Psychometrics play an important role in daily life. Whether you are a student, teacher, parent, psychologist, mathematician, or physicist; everyone has to deal with psychological tests. Psychological tests can have influenced your educational career, career, health, prosperity and so on. Psychometrics can even influence questions about life and death. For example in some countries, people with a severe cognitive impairment (significantly below average) can not receive the death penalty. But what is significantly below average? And how can we determine if the intelligence of an individual is below this limit? These kind of difficult questions can be answered through psychological tests. All in all, we can therefore say that psychometrics extends beyond psychological research. Psychometrics play a role in daily life, everyone has to deal with it.

How are psychological characteristics measured?

Psychologists use instruments to measure observable situations in the physical world. Sometimes psychologists measure a certain type of behavior solely because they are interested in that specific behavior in itself. But it is mainly behavioral scientists that measure human behavior to measure unobservable psychological attributes. We then identify a certain observable behavior and assume that this represents a certain unobservable psychological process, attribute or attitude. You must ensure that what you are measuring is also what you are aiming to measure. In social science, theoretical concepts such as short-term memory are often used to explain differences in human behavior. Psychologists call these theoretical concepts hypothetical constructs or latent variables. They are theoretical psychological properties, attributes, processes or states that cannot be observed directly. The procedures or actions with which they measure these hypothetical constructs are called operational definitions.

What is a psychological test?

According to Cronbach, a psychological test is a systematic procedure for comparing the behavior of two or more people. This test must meet three conditions:

1. the test must have behavioral samples;
2. the behavioral samples must be collected in a systematic manner and;
3. the purpose of the test must be to measure the behavior of two or more people (inter-individual differences).

It is also possible that we measure the behavior of an individual at different times, in which case we speak of intra-individual differences.    

Different types of testing

You can distinguish between different tests in the field of content. Which type of answer is used (open or closed) and which methods are used in the measurement. 

A distinction is also made between the different objectives of testing: the criterion referenced and norm referenced. Criterion referenced tests (also called domain referenced tests) are most common in situations where a statement must be made about a certain skill of a person. One pre-determined cutoff score is used to divide people into two groups: people whose score is higher than the cutoff score and people whose score is lower than the cutoff score.       

Norm-referenced tests are mainly used to compare the scores of a person with scores from the norm group. Nowadays, it is difficult to make a distinction between the benchmark tests and the benchmark tests. 

Another well-known distinction between tests is the distinction between the so-called speed tests (speed) and power tests. Speed tests are time-bound tests. It often happens that not all questions can be answered in a questionnaire. We look at how many questions you can answer correctly in the given time. Power tests are not time-bound tests. Here it is highly likely that one can answer all questions in a questionnaire. These questions often become more difficult and it is checked here how many questions people have answered correctly.        

Finally, the difference between reflective or effect indicators and formative or causal indicators is briefly discussed. An example of reflective / effect indicators are scores on intelligence or personality tests. These scores are usually considered as a reflection, or consequence, of a person's intelligence level. There are formative / causal indicators against this. Socio-economic status (SES) can, for example, be quantified by combining different indicators such as income, education level, and occupation. In this case, the indicators are not caused by SES. In contrast, the indicators are, in part, what SES defines. In this book the focus is on test scores, derived from reflective / effect indicators, which is typical of most tests and measurements in psychology.            

What is psychometrics?

With psychometrics, the focus is on the attributes of testing. Just as psychological tests are designed to measure psychological attributes of people, psychometry is the science where people are concerned with the attributes of psychological tests. There are three attributes that are important: the type of data (mainly scores), the reliability and validity of the psychological tests. Psychometrics is about the procedures with which test attributes are estimated and evaluated.

Psychometrics is based on two important foundations. The first foundation is the practice of psychological testing and measurement. The use of formal tests to measure skills of an individual (of any kind) goes back 2,000 years, or maybe even 4,000 years. Especially in the last 100 years there has been a huge increase in the number, type, and application of psychological tests. The second foundation is the development of static concepts and procedures. From the beginning of the nineteenth century, scientists became increasingly aware of the importance of static concepts and procedures. This led to an increase in knowledge about how quantitative data resulting from psychometric tests can be understood and analyzed. Pioneers in this field are Charles Spearman, Karl Pearson, and Francis Galton .       

Francis Galton was obsessed with measurements, mainly the so-called 'anthropometry'. Anthropometry contains measurements of human characteristics such as the size of the head, the length of an arm and the physical strength of the body. These properties have, according to Galton, psychological characteristics. He called these measurements of mental traits 'psychometrics'. Galton was primarily interested in the ways in which people differ. Galton's point of view was known as differential psychology , or the study of individual differences.   

Psychometrics is the collection of procedures that are used to measure variability in human behavior and subsequently combine these measurements into psychological phenomena. Psychometrics is a relatively young, but rapidly developing, scientific discipline. 

What are the challenges in psychometrics?

Many sciences are very similar but behavioral science has its challenges. 

One of those challenges is to try to identify and capture the important aspects of different types of human psychological attributes in a single number. 

A second challenge is participant reactivity. When participants know that they are being tested and they know why, this in itself influences the responses of the participant. For example, if a participant knows that it is being tested whether he or she is a racist but does not want this to appear in the test, this influences his or her reactions. Examples of participant reactivity are demand characteristics (influenced by what the participant thinks is the goal of the researcher), social desirability (responding to the wishes of the outside world), and malingering (wanting to leave a bad impression).       

A third challenge is that psychologists rely on so-called composite scores. This means that scores that have something in common are combined. For example, in a questionnaire with ten questions about extraversion, the scores on these questions are combined.  

A fourth challenge in psychological measurement is the problem of score sensitivity. Sensitivity refers to the possibility of a measurement to distinguish meaningful dimensions. For example, a psychologist wants to know if a patient's mood has changed. But if the psychologist uses an instrument that is not sensitive enough to measure small changes, the psychologist may miss important changes.  

The final challenge is the lack of attention to important information in psychometrics. Knowledge about psychometrics increases the chance of developments in testing. And test takers should at least use psychometrically good instruments. 

These challenges should make us aware of the data collected through psychological measurements. For example, we must be aware of the fact that participant reactivity can influence the reactions of the participants in a test.

What is the purpose of measuring in psychology? 

The theme that links the chapters of this book is related to the fact that the ability to identify and characterize psychological differences is the basis of all methods used to evaluate tests.

The purpose of measuring in psychology is to identify and quantify psychological differences that exist between people, over time or in different situations.

What is variability?

Variability (also called variance) is the difference within a set of test scores or between the values ​​of a psychological trait. Inter-individual differences are differences that occur between people. Intra-individual differences are differences within one person at different times. Individual differences are very important in psychological tests. Reliability and validity of tests depend on the ability of a test to quantify differences between people. All research in psychology and all scientific applications of psychology depend on the ability of a test to measure individual differences. It is important to know that every area of ​​scientific psychology depends on the existence and quantification of individual differences.

You can quantitatively display scores of a group of people or scores of one person at different times in a so-called distribution of scores. A distribution of scores is quantitative, because the differences between scores are expressed in figures. The difference between scores within a distribution is called the variability.

Calculating variability

To display a distribution of scores, you first have to calculate a few things. First of all, we calculate the average, since it is the most used, in addition to the mode and the median. Secondly, we calculate the variability, which consists of steps. These steps are as follows:

1. Calculate the mean by dividing the sum X by the total number of N people or things.
2. Calculate the deviation by displaying the difference between X and the average of X.
3. Calculate the squared deviation by squaring the deviation.
4. Calculate variance s² by dividing the sum of squared deviations by the total N.
5. Standard deviation √s² = calculate s by taking root of the variance.

Interpretation of the variance and standard deviation

• Is never less / lower than zero, or s² ≥ 0 and s ≥ 0.
• You can never interpret a single score as a large or small value.
• Comparison is only possible if two or more scores are based on the same measuring instrument / variable, for example IQ. After this you can also determine whether it is a large or small value.
• The variance and the standard deviation can only be used in certain concepts, for example in correlations or when you measure the reliability of scores.

Normal distribution

Distribution forms and normal distributions are qualitative because they represent the scores in a graphical way. The variable is placed on the x-axis, for example IQ scores from low to high. The proportions of the number of people who have achieved a certain score are displayed on the y-axis. A figure emerges from this: the normal distribution. This rarely (almost never) has a mirror shape. Usually the figures are either crooked to the right or crooked to the left. Skewed to the right means that there are more people who score low. Skewed to the left means that there are more people who score high.

What is covariability?

With a variance, the difference is calculated within one set of scores. With covariability, also called covariance, the difference of one set of scores is compared with the difference of another set of scores. In other words: with a covariance, the relationship between two variables is searched for, for example IQ and GPA. With a variance, one variable is used.

There are important characteristics with a covariance that clearly show the relationship between the two variables. The direction and strength of the relationship, but also the consistency between the two variables is important.

Direction and strength

The direction of the relationship between the two variables can have a positive or negative relationship. There is a positive (or direct) relationship when high scores on the first variable and high scores on the second variable occur at a time. A negative relationship exists when high scores for the first variable and low scores for the second variable occur. This can also be reversed, so low scores on the first variable and high scores on the second variable.

The strength of a connection is difficult to interpret.

Consistency

A strong relationship (positive or negative) between two variables shows that there is a high degree of consistency between them. If there is no clear relationship between two variables, then individual differences on one variable are inconsistent with individual differences on the other variable.

Variance is the variability of a single distribution of scores. Covariance is the variability of two distributions of scores. We have discussed the distribution of scores of a variance for this. We will calculate the distribution of covariance scores as follows:

1. Calculating deviations from variable X and from variable Y. You do this by calculating the difference between X and the average of X. You also calculate the difference between Y and the average of Y.
2. Calculate cross-products by multiplying the deviation of X and the deviation of Y by each other. A positive cross product can come out here, which means that the relationship between the variables is consistent. Or when the scores of an individual on both variables are consistent with each other, the individual scores either above the average on both variables or just below the average on both variables. A negative cross product can also be the result. This means that the coherence is uneven and therefore inconsistent. In other words, the individual scores below the average for one variable (so a negative deviation results from this), but on the other variable above the average (this results in a positive deviation).
3. Calculate covariance using a formula: Cxy = ∑ of the cross-products (multiplying deviation X by deviation Y) by the total N number of people or things.

The covariance provides clear information about the direction of the relationship but not about the strength of the relationship. Correlation coefficients provide clear information about the direction and strength of the relationship.

Variance-covariance matrix

The variance-covariance matrix is ​​always structured in a certain way, with a number of standard features:

1. Each variable has a row and a column.
2. The variances of the variables are shown in a diagonal line from top left to bottom right.
3. All other cells contain covarities between sets of variables.
4. The covariations are symmetrical. All values ​​below the diagonal are identical to the values ​​above the diagonal.

With a correlation, the value is easier to interpret than with a covariance. Correlation is always between -1 and +1. If the value is below zero, then the relationship between the two variables is negative. If the value falls above zero, it is positive.

Zero means that there is no correlation. The closer to zero the correlation falls, the poorer the relationship between the variables. You can also say that the coherence is inconsistent. The farther away from zero, the better / stronger the coherence between the variables. You can also say that the coherence is consistent. Correlation = Rxy = Cxy / SxSy.

Variance for compound variables / items is used when a psychological test contains a large number of variables / items. You first calculate the variances s² of the number of variables / items separately and add them together. You then calculate the correlation between the scores of the number of variables / items. You multiply this two times. In turn, you multiply this by the standard deviations of the number of variables / items (which you first calculated separately). In the end you add everything.

The total test score variance depends solely on item variability and the correlation between the item pairs. This is an important part of the dependency theory, which is discussed in a later chapter.

There are dichotomous reactions with binary items. This means that when answering a question you can choose from two answers (yes/no, agree/disagree, or 0/1). For example, we ask people to answer a yes or no to a question or we ask people whether they agree or disagree with the statement. It is also possible that certain scores are considered right or wrong. Whether we look at whether or not a certain disorder occurs. We often indicate this with codes, namely code 0 for a negative response (no, disagree, wrong, not true) and code 1 for a positive response (yes, agree, good, true). Code 1 is indicated by p = ∑X / N. Code 0 is indicated by q = 1-p.

You can also calculate the variance using p and q, namely: s² = p x q or p x (1-p). The maximum variance that you can get is 0.25; if p = q = 0.50 then s² = 0.50 x 0.50.

What should you pay attention to when interpreting test scores?

Problems arise when interpreting test scores. As an example:

• What is a high score and what is a low score?
• What does it mean if you score high or if you score low?

Consider for example a score of 35 on neuroticism. Is this a high score? And if this is a high score, what does that mean? Am I neurotic or not at all?

A frame of reference ensures that the figures and percentages are easy to interpret. You look at whether the scores fall above or below or even on the average score. You also view how many above or below the average score they fall (think of standard deviations). With this data you can calculate the so-called z-scores. A z-score shows how far above or below the average test score a score falls. Z-scores are easy to compare, even when two completely different variables / units of measurement are used within a test score. For example weight and optimism. Z = (X minus the average of X) / Sx (s = standard deviation of X). The z-score is expressed in the "number of standard deviations". An example, z = 0.5 or -0.5 this means that the score 0.5 standard deviations is above or below the average. This is very close. Another example, z = 2 or -2, this means that the score of 2 standard deviations is above or below the average. This is further away. A z-score distribution includes the following distribution: Z (0; 1) where 0 is the average and 1 is the standard deviation. Z scores say something about a score in relation to the rest of the group. It says how good or bad your score is compared to the average person but says nothing about your abilities in general. Correlation between variables using z-scores: Rxy = ∑ZxZy / N.

Z scores may be easy to compare, they are more difficult to interpret because many people are not familiar with concepts such as "standard deviations" or "distance to the average". Therefore, T-scores (standardized scores) are used with T (50; 10) where 50 is the average and 10 is the standard deviation. T = (z) times (s) + (average of X). Either T = z (10) + 50. Other means / standard deviations may also be given. Another way to interpret scores is in percentiles. An example: an individual has achieved a score of 194. The total number of people taking part in this test is 75. Only 52 people score lower than 194. So: (52/75) x 100 = 69%. You can interpret this as if the score of the individual falls in the 69th percentile and that this person scores higher than 69% of the other people who took the test.

What are normalized scores?

It is often assumed that a psychological trait is normally distributed, but this is not always the case and then a problem arises. It may be thought that a property (such as intelligence) is normally distributed, but the test data (IQ test) is not normally distributed. Researchers then assume that their theory is correct and that the test data (IQ scores) do not accurately reflect the distribution of the construct. Researchers have tried to solve this problem with the help of normalization transformations / area transformations. The scores are then converted to T-scores.

Variability (also called variance) is the difference within a set of test scores or between the values ​​of a psychological trait. Inter-individual differences are differences that occur between people. Intra-individual differences are differences within one person at different times. Individual differences are very important in psychological tests. Reliability and validity of tests depend on the ability of a test to quantify differences between people. All research in psychology and all scientific applications of psychology depend on the ability of a test to measure individual differences. It is important to know that every area of ​​scientific psychology depends on the existence and quantification of individual differences.

You can quantitatively display scores of a group of people or scores of one person at different times in a so-called distribution of scores. A distribution of scores is quantitative, because the differences between scores are expressed in figures. The difference between scores within a distribution is called the variability.

What is reliability?

Chapter 5 is about the reliability of a test. Reliability is the extent to which differences in the observed scores of the respondent concerned correspond with differences in his or her true scores. The smaller the difference, the more reliable.

According to the Classic Test Theory (CTT) the reliability can be determined on the basis of observed scores (Xo), true scores (Xt) and random scores (Xe). Random scores are also called measurement errors.

Other factors that cause differences between the observed and the true scores are called sources of error. These cause measurement errors, which create a contradiction between the observed and the true scores.

In addition to "sources of error", there are also temporary or transient factors that can influence the observed scores. Examples of this are the number of hours of sleep, emotional state, physical condition, gambling or misplaced answers. The latter means that if you know the correct answer, you still indicate the wrong answer. These temporary/transient factors decrease or increase the observed scores versus the reliable scores.

To find out whether the observed scores are a function of measurement errors or a function of reliable scores, two questions must be asked:

1. Which part of the observed scores is a function of reliable inter-individual or intra-individual differences?
2. Which part of the observed scores is a function of measurement errors?

In other words: Xo = Xt + Xe. You can say that the observed scores are determined by the true scores and the measurement errors. The smaller the value of Xe, the better. It seems that the measurement errors are random (at random), this means that they are independent of the true scores Xt. In other words, a measurement error affects both someone with a high true score and someone with a low true score in the same way and with the same amount. There are two characteristics:

• The average of all measurement errors within a test is zero.
• Measurement errors do not correlate with true scores, rte = 0.

Instead of saying that reliability depends on the consistency between differences in observed scores and differences in true scores, you can also say: reliability depends on the relationships between the variability of the observed score, variability of the true score, and variability of the measurement error score.

• Error score variance: Se² = ∑ (Xe minus average Xe) ² / N. The higher Se², the worse the measurement.
• True score variance: St² = ∑ (Xt minus average Xt) ² / N
• Observed score variance: So² = ∑ (Xo minus average Xo) ² / N. Or, So² = St² + Se².

This formula should actually be: So² = St² + Se² + 2rte * St * Se.

However, the true scores and the measurement errors are not correlated and therefore rte * st * se = 0. So there remains: So² = St² + Se².

What are the four types of reliability?

1. Reliability in terms of "proportions of variances"

Rxx (reliability coefficient) = St² / So²

Rxx = 0 means that everyone has the same true score. (St² = 0)

Rxx = 1 means that the variance of the true scores is equal to the variance of the observed scores. In other words: there are no measurement errors!

Here is an example of interpretation of Rxx:

Rxx = 0.48 or 48% of the differences in the observed scores can be attributed to the true scores. On the other hand, 1-0.48 = 0.52, so 52% of the differences can be attributed to measurement errors.

2. Reliability in terms of "lack of measurement error"

Rxx (reliability coefficient) = St² / So²

So² = St² + Se² (and therefore also: St² = So² - Se²)

Rxx = (So² - Se²) / So² = (So² / So²) - (Se² / So²)

In other words: Rxx = 1 - (Se² / So²): when (Se² / So²) is small, the reliability is high.

3. Reliability in terms of "correlations"

Rxx = Rot², where Rot² is the squared correlation between the observed scores and the true scores.

Rot = St² / (So * St) = Rot = St / So

Rot² = St² / So².

A reliability of 1.0 indicates that the differences between the observed test scores perfectly match the differences between the true scores. A reliability of 0.0 indicates that the differences between the observed scores and the true scores are totally contradictory.

4. Reliability in terms of "lack of correlation"

Rxx = 1 - Roe², where Roe² is the squared correlation between the observed scores and the error scores.

Roe = Se² / (So * Se) = Se / So

Roe² = Se² / So² so:

Rxx = 1 - Roe² = 1 - (Se² / So²).

If Roe = 0, then Rxx = 1.0

The greater the correlation between the observed scores and the error scores, the smaller Rxx. So reliability will be relatively high if the observed scores have a low correlation with the error scores.

How is the size of the measurement error expressed?

Although reliability is an important psychometric construct, it does not give a direct reflection of the magnitude of the measurement error of a test. Additional coefficients are therefore needed at this point. The standard measurement error displays the average size of the error scores. The greater the standard measurement error, the greater the average difference between observed scores and true scores, and therefore the lesser the reliability of the test.

Standard measurement error = sem

sem = So * √ (1 - Rxx)

If Rxx = 1 then Sem = 0, so: Rxx greater means sem smaller.
sem is never greater than So, so: greater means sem means greater
How is the theory of reliability translated into practice?

The theory of reliability is based on three terms: true scores, observed scores, and error scores. But in practice we do not know whether a score is actually the true score of an individual. We also do not know to what extent measurement errors influence the response of an individual. How then do we translate the theory of reliability into practice?

Although we cannot determine with certainty what the reliability or standard measurement error of a test is, advanced methods have been developed to estimate it. Examples of such techniques are giving two versions of the test, doing the same test twice and so on. In this section, four methods are discussed to estimate the reliability and standard measurement error of a test:

1. Parallel testing;
2. the tau equivalent test model;
3. essentially tau equivalent test model;
4. congeneric test model.

Each model offers a perspective on how two or more tests are the same.

1. Parallel tests

We speak of parallel tests when two (or more) tests, in addition to the basic assumptions of classical test theory, meet the following three assumptions:

1. The two tests have the same error variance (se12 = se22).
2. The intercept between the true scores on both tests is 0 (so a = 0, in Xt2 = a + b (Xt1)).
3. The slope between the true scores on both tests in 1 (so b = 1, in Xt2 = a + b (Xt1)).

These assumptions have six implications:

1. This implies that the true scores of test 1 are identical to the true scores of test 2 (Xt1 = Xt2).
2. Derived from this it means that in case the true score of each participant on test 1 is equal to the true score on test 2, the two sets of true scores correlate perfectly with each other (rt1t2 = 1).
3. The variances of the value scores of tests 1 and 2 are identical (st12 = st22).
4. The average of the true scores of test 1 is equal to the average of the true scores of test 2.
5. The variance of the observed scores of test 1 is equal to the variance of the observed scores of test 2. And finally, sixth, the reliability of the tests is the same (R11 = R22).
6. When the scores of two tests meet all these assumptions and implications, we speak of parallel tests.

Finally, according to the KTT, there is one further implication that follows from the above: the correlation between parallel tests equals reliability. In formula form: r0102 = R11 = R22. In other words, when two tests are actually (perfectly) parallel, the correlation between the two tests is therefore equal to the reliability of both tests.

The correlation between parallel tests can also be calculated based on the variances of the true and observed scores: r0102 = st2 / so2.

2. The tau-equivalent test model

In addition to the standard assumptions of classical test theory, the tau-equivalent test model is based on the following two assumptions:

1. The intercept between the true scores of both tests is 0 (so a = 0, in Xt2 = a + b (Xt1)).
2. The slope between the true scores on both tests in 1 (so b = 1, in Xt2 = a + b (Xt1)).

These two assumptions are the same as the assumptions of parallel tests. The difference lies in the first additional assumption: the tau-equivalent test model does not state the assumption of appropriate error variances. This leads to four implications (the first four that we discussed in parallel testing).

The less strict assumptions mean that the correlation between tau-equivalent tests is not a valid estimate of the reliability. This is in contrast to parallel tests, where the correlation between the tests is therefore a valid estimate of the reliability.

3. The essentially tau-equivalent test model

In addition to the standard assumptions of classical test theory, the essentially tau-equivalent test model is based on one additional assumption:

The slope between the true scores on both tests in 1 (so b = 1, in Xt2 = a + b (Xt1)).
This leads to two implications (the first two discussed in parallel tests), or: rt1t2 = 1 (the correlation between the tests is perfect), and st12 = st22 (the variances of the true scores of both tests are equal).

4. Congeneric test model

The last model is the congeneric test model. According to this model, only the assumptions of classical test theory are accepted. This results in a single implication, namely that the correlation of the true scores between the tests is equal: rt1t2 = 1. This model is therefore the most strict and the most general model. Although this model is more often applicable (this model is conditional for more districted models), it offers limited possibilities for estimating reliability.

What is the 'Domain Sampling Theory'?

According to this theory, reliability is the average size of the correlations between all possible pairs of tests with N items selected from an area ("domain") of test items. The logic of this theory is the foundation of the generalizability theory, this will be discussed extensively in chapter thirteen.

Chapter 5 is about the reliability of a test. Reliability is the extent to which differences in the observed scores of the respondent concerned correspond with differences in his or her true scores. The smaller the difference, the more reliable.

This chapter explains how reliability and measurement errors affect the results of behavioral research. Awareness of these effects is crucial for behavioral research.

Which two sources of information can help evaluate an individual test score?

There are two important sources of information that can help us evaluate an individual test score. The first is a point estimator. This is a value that is interpreted as the best estimate of someone's score on a psychological trait. The second is a confidence interval, which gives an area with values ​​in which the true score of a person lies. If the true score has a large confidence interval, we know that the observed score is a poor point estimator of the true score.

Point estimates

Two types of point estimates can be taken from an individual observed score. The first point estimator is based on the observed test score only. When a test subject takes the test at a certain moment, an observed score is obtained. This is then an estimate of the true score. The second point estimator also takes the measurement error into account. By estimating with the score of the first test what the test subject will score in the second test, an adjusted true score can be estimated based on this estimate. When a test subject takes the test for a second time, the second time the score will be closer to the group average. This is called regression to the mean. This prediction is based on the logic of the classical test theory and the random measurement error. An estimate of the adjusted true score shows the difference between someone's observed score on the first test and the observed score on the second test. The magnitude and direction of the difference depends on three factors:

1. The reliability of the test scores.
2. The magnitude of the difference between the original observed test score and the average of the test scores.
3. The direction of the difference between the original score and the average of the test scores. The following formula is used to estimate the adjusted true score.

X_est = X_avg + Rxx(X_o – X_avg)

X_est is the estimate of the adjusted true score, X_avg is the average of the test score, R_xx is the reliability of the test and Xo is the observed score. The reliability of the test influences the difference between the estimated true score and the observed score. With a smaller reliability, the difference between the estimated true score and the observed score becomes larger. The observed score itself also influences the difference between the estimated true score and the observed score. The difference will be greater with more extreme observed scores.

One reason for not calculating the estimated true score is that an observed score is already a good estimator of the psychological characteristic and there can be little reason to correct it. A second reason is that the estimated value does not always lead to a regression to the average.

Confidence intervals

Confidence intervals represent the accuracy of the point estimator of an individual true score. The accuracy of the confidence interval and the reliability have a link due to the standard measurement error (se_m).

se_m = s₀ √ (1 - Rxx)

The greater the standard measurement error, the greater the average difference between observed scores and true scores. For the calculation of a 95% confidence interval around the standard measurement error, the following formula applies:

95% confidence interval = Xest ± (1.96) (sem)

Xest is the adjusted true score (that is a point estimate of the true score of an individual), sem is the standard measurement error of the test scores and 1.96 (the z-score) indicates that we calculate a 95% confidence interval. The interpretation of a confidence interval is that we can say with 95% certainty that the true score can be found somewhere in the confidence interval. Tests with a high reliability will require a smaller confidence interval than tests with a lower reliability. Reliability affects the confidence, accuracy and precision with which a person's true score is estimated.

Confidence intervals can be calculated in different ways and with different sizes (95%, 90%, etc.)

The intervals can be calculated with the standard measurement error or the standard estimation error (which is also influenced by reliability). The estimates of the true scores, and the confidence intervals that go with them, are important in making decisions. And reliability plays a major role in this.

On which two factors does the correlation of observed scores from two measurements depend?

According to the classical test theory, the correlation of the observed scores of two measurements (r_xoyo) depends on two factors: the correlation between the true scores of the two psychological constructs (r_xtr_yt) and the reliability of the two measurements (R_xx and R_yy).

r_xoyo = r_xtyt * √ (R_xx * R_yy)

The correlation between two sets of observed scores is:

r_xoyo = c_xtyt / s_xosyo

We can calculate the observed standard deviation with the reliability and the standard deviation of the true scores. See below:

s_xo = s_xt / √R_xx and s_yo = s_yt / √R_yy

The classical test theory shows that the correlation between two measurements is determined by the correlation between psychological constructs and the reliability of the measurements.

The measurement error suppresses the correlation between measurements

There is a difference between the correlation of the observed scores and the correlation of the true scores. This has four important consequences:

The intervals can be calculated using the standard measurement error or the standard estimation error (which is also influenced by reliability). The estimates of the true scores, and the confidence intervals that go with them, are important in making decisions. And reliability plays a major role in this. On which two factors does the correlation of observed scores from two measurements depend? According to the classical test theory, the correlation of the observed scores of two measurements (rxoyo) depends on two factors: the correlation between the true scores of the two psychological constructs (rxtryt) and the reliability of the two measurements (Rxx and Ryy) . rxoyo = rxtyt * √ (Rxx * Ryy) The correlation between two sets of observed scores is: rxoyo = cxtyt / sxosyo We can calculate the observed standard deviation with the reliability and the standard deviation of the true scores. See below: sxo = sxt / √Rxx and syo = syt / √Ryy The classical test theory shows that the correlation between two measurements is determined by the correlation between psychological constructs and the reliability of the measurements. The measurement error suppresses the correlation between measurements There is a difference between the correlation of the observed scores and the correlation of the true scores. This has four important consequences:

1. The observed correlations (between measurements) will always be weaker than the correlations of the true scores (between psychological constructs). This is because measurements will never be perfect and imperfect measurements make the observed correlations weaker.
2. The degree of weakening depends on the reliability of the measurements. Even if only one of the tests has low reliability, the correlation of the observed scores becomes a lot weaker compared to the correlation of the true scores.
3. Error limits the maximum correlation that can be found. As a result, the observed correlation of two measurements can be lower than expected.
4. It is possible to estimate the true correlation between two constructs. Researchers can estimate all parts of the formula except for the correlation of the true scores. When converting the formula, it results in the following :

   r_xtyt = r_xoyo / √R_xx * R_y

   This formula is called the correction for attenuation, because researchers can see what the correlation would be if it were not influenced by weakening. The estimated correlation has a perfect reliability and with a perfect reliability the observed correlation is equal to the true correlation.

In addition to reliability, what else should you pay attention to when looking at the results of a study?

Because the measurement error reduces the observed correlation, this has disadvantages for interpreting and leading the research. Results must always be interpreted with the help of reliability. An important result of a study is the effect size. Some effect sizes show the extent to which the variables are interrelated and others show the magnitude of the differences between groups.

An example of an effect size that shows the extent to which two variables are interrelated is the correlation coefficient. High reliability results in larger observed effect sizes and lower reliability reduces the observed effect sizes. There are three common effect sizes that are used in studies: correlations, Cohen's d and N2. These effect sizes are each used in different analytical situations. 1. Correlation is usually used to represent the relationship between two continuous variables. 2. Cohen's d is usually used when looking at the relationship between a dichotomous variable and a continuous variable. 3. N² is usually used when looking at the relationship between a categorical variable with more than two levels, and a continuous variable.

A second important result of a study is statistical significance. Statistical significance provides certainty of a result. If a result is statistically significant then it is seen as a real find and not just a fluke. With statistical significance, a clear difference is demonstrated. The observed effect has a major influence on statistical significance. If the effect size becomes larger, the test is rather statistically significant.

The effect of reliability on effect size and statistical significance is very important when looking at the results of a study.

Including reliability when drawing psychological conclusions from an investigation has three important implications. The first is that researchers should always include the effects of reliability on the results obtained when they interpret effect sizes and statistical significance. The second is that researchers must use measurements that have high reliability. That way the problem of weakening can be kept to a minimum. Yet there are two reasons why a researcher sometimes uses measurements with low reliability. The first reason is that the interest may lie in an area where it is very difficult to obtain high reliability. A second reason may be that researchers work with a low reliability, because a measurement method with a higher reliability has not been searched for long enough. It can take a lot of time, money and effort to find a good method with high reliability. Researchers make the assessment of the effort they want to put in and the reliability that they want to achieve. The third implication of including reliability is that researchers should report reliability estimates of their measurements. This is necessary because the readers must be able to interpret the results.

What should you pay attention to in the construction and improvements of tests?

With test construction and improvement, attention is drawn to the consistency of the test parts and then the items are mainly looked at. Test developers test the items from a test and see which items can be removed or which have to be strengthened to improve the quality of psychometric tests.

To see if an item contributes to internal consistency, the item average is looked at, the item variance and the item discrimination. It is important to know that the procedures and concepts described below must be performed for each dimension measured by the test. So in a one-dimensional test, the following analyzes would be performed on all test items together as one group. And with a multidimensional test, the following analyzes would be performed separately for each dimension.

Item discrimination and other information concerning internal consistency

An important factor for the reliability of internal consistency is the extent to which the test items are consistent with each other. The internal consistency has an intrinsic link with the correlations between the items. With a low correlation, one item has little consistency with the other items and the internal consistency decreases.

To calculate the correlation between items, SPSS can be used to look at the "inter-item correlation matrix", but because many tests consist of many items, this is not the most convenient method. Item discrimination is the extent to which an item distinguishes between people who score high on a test and people who score low on a test. High discrimination values ​​are required for good reliability. There are several ways to calculate an item discrimination. One is the item-total correlation. We can calculate a total score and then calculate the correlation between an item and the total score. This item-total correlation shows how large the difference in responses is at the item compared to how large the difference in responses is in total. A high item-total correlation indicates that the item is consistent with the test as a whole.

With SPSS it is indicated as corrected item-total correlations and then the correlation with the total score is calculated for each item. It is "corrected" because the item itself does not count towards the total score. Another way of item discrimination for binary items is the item discrimination index (D). This compares the proportion (p) of people who scored high on the test and answered the item correctly, with the proportion (p) of people who scored low on the test and answered the item correctly. The proportion of well-answered questions in the group is calculated for those two groups. The difference between the two groups can then be calculated by subtracting the proportion of the lowest group from the proportion of the highest group.

D = p_high - p_low

Items with high D scores are better for internal consistency. SPSS has two other ways to look at the internal consistency of a test, namely the squared multiple correlation and the Cronbach's Alpha if item deleted. The latter gives the correlation of the total test if one item is removed from the list.

Item variance and Item difficulty (mean)

The item's mean and variance are important factors that can influence the quality of a psychometric test. They can contribute to how consistent an item is with the rest of the items. This is important for the reliability of the test. A variable needs variability to be able to correlate with another variable.

If all test subjects answer the same, there is no variability. Variability is required with good reliability.

A link between the item variability and the psychometric quality can be made by the item average. The item average can say something about the item variability. An item with limited variability makes little contribution to psychometric quality. The averages can also be seen as a difficulty. If more people have a good answer to one question than another, the difficulty is different. For example, if the average is 0.70, it means that 70% of people have answered the item correctly. The classical test theory suggests that binary test items must have an average of 0.50 so that all items have maximum variability.

This chapter explains how reliability and measurement errors affect the results of behavioral research. Awareness of these effects is crucial for behavioral research.

In the previous chapter we discussed the conceptual framework of validity, where we could identify five types of proof of validity. One of the types of evidence was convergent and divergent validity: the extent to which test scores have the "right" pattern of associations with other variables. This is discussed further in this chapter. 

What is a nomological network?

Psychological constructs are laid down in a theoretical context. The basis of the construct has connections with the basis of other psychological constructs. The connection between the construct and other related constructs is called a nomological network . According to this network, measurements from one construct would be strongly associated with some other constructs, but weakly correlate with measurements from other constructs. For validity it is important that the test scores correspond as much as possible with the expected associations.  

What methods are there for evaluating convergent and discriminant validity?

There are four methods used to look at convergent and discriminatory associations. The following four methods are common methods for evaluating convergent validity and discriminant validity:

1. Focus on certain associations;
2. correlation sets;
3. multitrait multimethod matrices;
4. quantify construct validity (QCV).

These four methods are discussed in this section. 

1. Focused associations

With some measurements it is fairly clear which specific variables are related to it. For the validity of the interpretations, the relationship between the test scores and those specific variables can then be examined. When the test scores are highly correlated with the variables, there is a strong validity and if the correlations are low, the validity can be called into question. Test developers get more confidence in the test if the correlation with relevant variables is high. These correlations are called validity coefficients. The quality of a test is higher when the validity coefficients are high.  

A process in which all validity coefficients are tested in multiple studies is called validity generalization. The most validity proof comes from relatively small studies. The correlation is then calculated between the test scores that have been measured and the scores on the criterion variables. The small studies are often done and can also be used, but it also has a disadvantage. If a test has been carried out at one location or at a certain population, which results in an excellent validity score, this does not necessarily mean that it will be the same at another location or at another population.

Studies that look at the validity generalization are meant to investigate the usefulness of the test scores. This type of research is a kind of meta-analysis, they combine the results of several smaller studies into one large analysis. There are three important things about validity generalization:

• It can reveal a general level of the predicted validity of all smaller studies.
• It can reveal the degree of variability between the smaller studies.
• It deals with the source of the variability between the smaller studies. Further analysis of small studies may explain differences between these studies.

2. Sets of correlations

The nomological network of a construct can have associations with other constructs of different levels. As a result, when evaluating convergent validity and discriminant validity, a large amount of criterion variables can be considered.

The researchers usually calculate all correlations between the variable and the criterion variables. From there, a subjective look is taken at which correlations and therefore which criterion variables are relevant. So which criterion variables go into the nomological network. This approach to evaluating validity is common among researchers. First the researchers collect as much data as possible and take many relevant measurements. Then the correlation patterns are examined and the patterns that mean something for the test are included in the test.

3. Multitrait multimethod matrices

Campbell and Fiske have developed the multitrait multimethod matrix (MTMMM) from the conceptual basis of Cronbach and Meehl. With the MTMMM analysis, construct validity is obtained by means of measurements of multiple properties and several different methods are used. The purpose of the MTMMM analysis is to get a clear evaluation of the convergent validity and the discriminant validity. Two important sources of variance can influence the correlations between the measurements. These are the property variance and the variance in the methods. A high correlation between two properties can mean that they share property variance. A correlation can also be high because both properties were measured with the same method. They then have a shared method variance. This can cause a correlation to come out while the properties have no correlation at all. But because it was done with the same test, there is a correlation by the way the subject thinks. For example, the subject can make both tests with low self-esteem, it makes sense that there can be a correlation with two traits. A high correlation can therefore indicate sharing of the property variance but it can also indicate a shared method variance. A correlation can also be weak because two different methods have actually been used, while the properties may actually have a correlation. This makes it difficult to interpret construct validity. Each correlation is a mix of property variance and method variance. The MTMMM analysis organizes relevant information and makes it easier for researchers to interpret the correlations.

An MTMMM analysis must be a good test of different correlations that represent different property and method variances. This is possible, for example, with two correlations:

• A correlation in which the same property was tested with two different measurements.
• A correlation in which different properties were tested with one type of measurement.

The first correlation is expected to be strong and the second correlation to be weaker. When the method variance is included, it can be expected that the first correlation is weaker and the second correlation stronger.

Campbell and Fiske (1959) have derived four types of correlations from the MTMMM:

1. Heterotrait-heteromethod correlations: these are different traits, measured with different methods.
2. Heterotrait-monomethod correlations: here different properties are subjected to the same method.
3. Monotrait-heteromethod correlations: the same property (construct) is measured with different methods.
4. Monotraot-monomethod correlations: a property is measured with a method. These correlations represent the reliability; the correlation of the measurement with itself.

Evaluating the construct validity, the property variance and the method variance with the different correlations can be looked at in a clear way with the MTMMM analysis. The convergent validity can be found by looking at the monotrait-heteromethod correlations. The correlations of the measurements that share the property variance and do not share a method variance must be greater than the correlations of the measurements that share no property variance and no method variance. Also, the correlations of the measurements that share the property variance and do not share method variance must be greater than the correlations of the measurements that do not share the property variance and the method variance.

Today, improvements for the MTMMM analysis are still being looked into. Despite the familiarity of the MTMMM analysis, it is not often used.

4. Quantifying Construct validity (QCV)

With this method, researchers quantify the extent to which the theoretical predictions for the convergent and discriminant correlation fit with the actual obtained correlations. So far, evidence for convergent validity and discriminant validity has been primarily subjective. The one may find the correlation strong while the other experiences it as less strong. The QCV procedure has been developed to obtain as objective and precise validity as possible. This makes the fourth method different from the other three methods.

The effect measures are taken first from the QCV analysis. So here we look at the extent to which the actual correlations correspond to the predicted correlations. These effect measures are r alerting -CV and r contrast -CV. called. High and positive correlations mean that the actual convergent and discriminant correlations are very similar to the predicted convergent and discriminant correlations. Secondly, a test of statistical significance follows from the QCV analysis. The statistical significance examines whether the correspondence between the two correlations has not happened by chance.  

The QCV analysis takes place in three phases:

1. Researchers make clear predictions about the expected convergent and discriminant validity correlations. Good consideration must be given to the criteria attached to the measurements and a prediction must be made of each correlation relevant to the test.
2. In the second phase the researchers collect the data and the actual convergent and discriminant correlations are calculated. These correlations show the actual correlations between the variable that we are interested in and the criterion variables.
3. In the third phase, the extent to which the predicted correlations and actual correlations are matched is quantified. If the correlations are well matched, this means a high validity. If the correlations do not match well, this means a low validity. The test is presented with two types of results, the effect measures and the statistical significance. For the effect measure r alerting -CV , the correlation between the predicted correlations and the actual correlations is calculated. A high, positive correlation means that the predicted correlations and the actual correlations are well matched. The same applies for the r constrast -CV , the greater the correlation, the better it is for the convergent and discriminant validity. The r constrast -CV is similar to the r alerting -CV , but it is corrected for the mutual correlations between the criterion variables and the absolute level of the correlations between the main test and the criterion variables. The statistical significance is also examined in the third phase of the QCV analysis. This examines the size of the test and the amount of convergent and discriminant validity. With a z- test it is then calculated whether the correlations were not obtained by chance.   

The QCV approach can be a good approach, but it is not perfect. The effect measures may have low values ​​due to incorrect predictions, while the proof of validity may be high. A wrong choice can also be made in choosing the criterion variables. Another criticism is that researchers had high values ​​for the effect measures but that predicted convergent and discriminant correlations did not match well with the actual convergent and discriminant correlations.

Multiple strategies can be used in the analysis of tests. Although the QCV analysis is not perfect, it still has advantages over the other methods. First, the QCV analysis allows researchers to look closely at the pattern of convergent and discriminant validity that would theoretically make sense. Second, it allows the researchers to make explicit predictions about the associations with other variables. Thirdly, the QCV analysis focuses on the variable of interest. Finally, it provides an interpretable value that reflects the extent to which the actual outcomes match the predicted outcomes, and the QCV analysis also contains statistical significance.

Which factors can influence the validity coefficients? 

In the previous section we discussed strategies that can be used to accumulate and interpret evidence for convergent and / or divergent validity. All these strategies are determined to a greater or lesser extent by the size of the validity coefficients. Validity coefficients are static results that represent the degree of associate between a test and one or more criterion variables. It is important to be aware of the factors that can influence the validity coefficients. For that reason we discuss those factors in this section.     

The associations between constructs

A factor that influences correlation is the true association between two constructs. If two constructs are strongly associated with each other then a high correlation is likely to result. With predictions, a correlation is expected, because it is thought that there is a connection between the constructs.

The measurement error and reliability

Measurement errors can influence the correlations and therefore also the validity coefficients. The correlation between testing of two constructs is:

r_xoyo = r_xtyt √( R_xx * R_yy)  

r_xoyo here is the correlation between the two tests, r_xtyt here is the actual correlation between the two constructs, R_xx is the reliability of the test variable and R_yy is the reliability of the criterion variable. To evaluate the convergent validity, researchers must compare the correlations with the expected correlations. When evaluating the validity correlation, one must take into account the fact that two reliabilities are used. The reliability of the test and the reliability of the criterion test. The criterion test can have a low reliability, which means that the validity is also lower. If the reliability of one of the tests is low, this can be tackled in two ways. The first is to give less weight to the test with low reliability when assessing validity. The second is to adjust the validity coefficient by means of the correction for attenuation. If you want to adjust the coefficient for the reliability of one test, then this form of the formula can be used:    

r_XY-adjusted = r_XY-original / √R_yy  

r_XY-original is the original validity correlation, R_yy is the estimated reliability of the criterion variable and r_XY-adjusted is the adjusted validity correlation.  

A limited range

A correlation coefficient shows the covariability between two distributions of scores (we discussed this earlier in Chapter 3). The amount of variability in the distributions can influence the correlations between the two sets of scores. The correlation can therefore be limited by a limited range in both distributions and as a result it provides relatively poorer proof of validity.

There are no clear, simple guidelines to identify the degree of range limitation; it especially requires caution (careful thought) and attention from the researchers. Importantly, such as knowledge about relevant tests and variables. For example, a researcher must thoroughly analyze whether all observed scores fall within the range that is (theoretically) possible for that construct. A commonly used method to assess the degree of range limitation is by looking at the degree of convergent and discriminant validity. The correlations are used to look at the quality of the psychological measurement. With strong correlations expected, the convergent evidence is examined. The correlations can be lower due to the influence of a limited range.    

The relative proportions

The skew of the distributions of the scores also affects the size of the validity coefficient. If the two variables that correlate with each other have a different skew then the correlation between these variables will be reduced. So if research is being done into a variable with a very skewed distribution, then it is possible that a relatively small validity coefficient will come out. 

The formula for the correlation between a continuous and a dichotomous variable ( r CD ) is:

r_CD = c_CD / s_Cs_D  

c_CD is the covariance between the two variables, s_C is the standard deviation of the continuous variable and s_D is the standard deviation of the dichotomous variable. Due to the proportion of observations in the two groups with the dichotomous variable, the covariance and the standard deviation are directly influenced. The covariance for this is:   

c_CD = p₁p₂ (C_2avg - C_1avg )   

p₁ is the proportion of participants in group 1, p₂ is the proportion of participants in group 2, C_1avg is the average of the continuous variable in group 1 and C_2avg is the average of the continuous variable in group 2. The standard deviation of the dichotomous variable is the second term that is influenced by the proportion of observations. The formula for this is:    

s_D = √p₁p₂ 

The calculation for the correlation can be converted to show the direct influence of the relative proportions:

r_CD = √p₁p₂ (C_2avg - C_1avg ) / s_C   

This formula shows the influence of group promotions on the validity correlations. When the validity coefficient is based on a continuous variable and a dichotomous variable, the validity can be influenced by differences and the size of the groups. The validity can be lower if there is a difference in the size of the groups.

The method variance

This was previously discussed in the MTMMM analysis. Correlations between two different methods are smaller than correlations between measurements from one method. If only one method is used , there is a good chance that the correlation is greater, because it also contains a shared method variance.

Time

Validity coefficients based on correlations calculated from measurements at different times are smaller than correlations calculated from measurements at the same times. And longer periods between two moments in time will produce smaller predictive validity correlations.

The predictions of some events

An important factor that can influence the validity coefficient is whether the criterion variable has been one event or a list of multiple events. One-off events are more difficult to predict than a list of multiple events. It is more likely to obtain large validity coefficients when the criterion variable is based on the listing of multiple events.

How can you interpret the validity coefficient? 

Once the validity coefficient has been determined, it must be decided whether it is high enough for convergent evidence, or low enough for certainty of discriminatory validity. Although there is a precise way to quantify the relationship between two measurements, it will not always happen intuitively. Especially for inexperienced researchers, evaluating validity can then be problematic. He or she does not know well when a correlation is strong or weak.

The explained variance and squared correlations

In psychological research it is common to use squared correlations. These show the proportions of variance in one variable, which are explained by the other variable. The explained variance interpretation is attractive, because earlier claims say that research in general is about measuring and understanding variability. The more variability you can explain, the better it can be understood. For the explained variance, the variance analysis is applied (ANOVA).

There are three reasons for criticizing the squared variance:

1. In some cases it is technically wrong.
2. Some experts say that the variance itself is a non-intuitive metric. For a measurement of differences in a set of scores, the variance is based on the squared deviations from the mean.
3. Squaring the correlation can make the relationship between two variables appear smaller.

The squared correlation approach for interpreting the validity coefficients is widely used, but it can also be misleading. It also has a number of technical and logical problems.

Estimating practical effects

One way to interpret the correlation is to estimate how much effect it has in real life. The greater the correlation between the test and the criterion variable, the more successful it can be used in decisions about the criterion variable.

Four procedures have been developed to properly predict the correlations:

1. Binominal Effect Size Display (BESD)

This procedure has been developed to show the practical consequences of using correlations to make decisions. With the BESD you can see how many predictions are successful and how many predictions will not be made successful based on the correlation. That can be viewed with a 2x2 model. The following formula is used to predict the number of people in a cell of the table:

Cell A = 50 + 100 (r / 2)

r is here the correlation between the test and the criterion. The following formula applies to cell B:

Cell B = 50 - 100 (r / 2)

Cell C has the same formula as Cell B and Cell D has the same formula as Cell A.

By putting the validity correlation in a kind of table and converting the numbers into successful predictions, it is easier to see whether the test has a good validity. The criticism is that the test is only suitable if as many people score high as low. And it is made for a situation in which half of the sample is 'successful' on the criterion and the other half unsuccessful. The BESD assumes the same relative proportions.

2. Taylor-Russel Tables

These tables can be used when the assumption of equal proportions is unfounded.

These tables give the chance that a prediction based on an 'acceptable' test score will lead to a successful implementation of the criterion. The Taylor-Russel tables, like BSED for the test and the outcomes, have dichotomous variables. The difference with the BSED is that the Taylor-Russel tables can make decisions based on different proportions. For the Taylor-Russel tables, we need to know the size of the validity coefficient, what the selection proportion is and what the successful selection proportion is if the selection would have been made without the test.

3. Utility Analysis

The utility analysis formulates validity in terms of costs versus benefits. Researchers must assign valid values ​​to various aspects of testing and decision making in the process. First, the benefit of using this test to make decisions compared to other methods that can be used must be estimated. The researcher must then estimate the costs (disadvantages) if this test is used to make a decision.  

4. Sensitivity and specificity

This is especially useful for tests designed to identify a categorical difference. The ability of the test to make the correct identifications with regard to the categorical difference can then be evaluated. An example is a diagnosis where the disorder may be present or absent. There are four possible outcomes:

1. True positive, the test provides a good identification where the disorder is really present (true positive).
2. True negative, the test gives a good identification where the disorder is not present (true negative).
3. False positive, the test indicates that the disorder is present while in reality it is not(false positive).
4. False negative, the test indicates that the disorder is absent while it is actually present(false negative).

Values ​​of sensitivity and specificity are values ​​that summarize the proportions of good identifications. The sensitivity let the chance to see someone with a disorder correctly identified by the test. Specificity shows the probability see someone who the disorder is not correctly identified by the test. In reality one can never know if someone has a disorder, but it is a guideline that is trusted. We will illustrate both concepts with an example (see the table below).

Table 1.

In this example there are 80 true positives, 120 untrue positives, 20 untrue negatives and 780 true negatives. The sensitivity can be calculated by: 

Sensitivity = true positives / (true positives + false negatives)

In the example this amounts to: 80 / (80 + 20) = 80/100 = .80. The sensitivity (the proportion of individuals with the disorder who were correctly identified by the test) is therefore .80. In other words, 80% of people who actually have the disorder are also identified as such by the test. Although there is a high (ie good) sensitivity here, sensitivity alone is not sufficient to claim that there is a high validity. There must also be a high degree of specificity.  

Specificity = true negatives / (true negatives + false positives)

In the example this amounts to: 780 / (780 + 120) = 780/900 = .87.

The proportion of people without a disorder who are also identified as such by the test (ie as the absence of the disorder) is .87. In this example, there is therefore a high degree of sensitivity and specificity. Based on this, we can state that it is plausible that there is a high validity.   

Finally, on the basis of sensitivity and specificity, you can calculate the correlation between the test results and the clinical diagnosis using the following formula:

r = (TP * TN - FP * FN) / √ ((TP + FP) * (TP + FN) * (TN + FP) * TN + FN))  

with TP = true positives (true positive), TN = true negatives (true negatives), FP = false positives (false positives) and FN = false negatives (false negatives).

In our example, the correlation is:

r = (80 * 780 - 120 * 20) / √ ((80 + 120) + (80 + 20) + (780 + 120) + (780 + 20)) 

r = 60000/120000

r = .50

The correlation between the test results and the clinical diagnosis is therefore .50. 

Guidelines and standards in the field

Another way to look at the correlations is to evaluate the context. Different requirements apply in one research field than in another. Things are found in physical science that are much more powerful than findings in behavioral science. According to the guidelines of Cohen (1988), correlations of 0.10 are considered small in psychology, correlations of 0.30 are considered medium and correlations of 0.50 are considered large. Nowadays Hemphill (2003) has made new guidelines. Now a correlation below 0.20 is small, between 0.20 and 0.30 is medium and above 0.30 the correlation is large.

Statistical significance

Statistical significance is an important part of inferential statistics. Inferential statistics are procedures that help us make decisions about populations. Most studies have a small number of participants. Most researchers use this small number of participants as an example for the entire population. And assume that this data is a good representation of the data that they would receive if they examined the entire population. Nevertheless, the researchers are aware that it is not possible to just say things about the entire population as is the case in the sample.  

The inferential statistics are used to gain more confidence in statements about an entire population if only samples have been used. When a sample is statistically significant, it is representative of the population. If there is no statistical significance, then the correlations cannot accurately represent reality and the correlations may therefore have been obtained by chance. It is therefore logical that many researchers find statistical significance very important.

When evaluating convergent validity, it is expected that the validity coefficients are statistically significant. When evaluating discriminant validity, it is expected that the validity coefficients are not statistically significant. With statistical significance, the question arises: do we believe that there is a validity correlation (not zero) in the population from which the sample was taken? And how sure are we that it is? And are we sure enough to conclude that? Two factors that influence the questions are the size of the correlation in the sample and the size of the sample. Confidence rises when the correlation in the sample is not zero, but it can happen that the correlation in a sample is not zero while it can be zero over the entire population. A second factor is the size of the sample. The greater the number of test subjects, the greater the confidence in the sample. So larger correlations and larger samples increase the chance that a test is statistically significant.

Are we sure enough that the correlation in the population will not be zero? Researchers have found that a test with a confidence interval of 95% has statistical significance. So a test is statistically significant if there is a 5% chance of being wrong (this is the alpha level). It is possible that there are low correlations and that the test is nevertheless statistically significant or that there are high correlations and that the test is not statistically significant.

A non-significant convergent validity correlation may be due to a small correlation or a small sample. If the correlation is small, then this evidence is against the convergent validity of a test. If the correlation medium is to large, but the sample is small, then it is not necessarily the case that the convergent validity is poor. In this case the examination is poor because the sample was too small.

With discriminant validity, a high correlation provides evidence against the discriminant validity. A significant discriminant validity correlation can arise because the correlation is large or because the sample is large. If the correlation is large then this evidence is against the discriminant validity of a test. If the correlation is small, but the sample is large, then it does not necessarily mean that the discriminant validity is poor. In such cases the statistical significance says nothing and it is better to ignore it.

In the previous chapter we discussed the conceptual framework of validity, where we could identify five types of proof of validity. One of the types of evidence was convergent and divergent validity: the extent to which test scores have the "right" pattern of associations with other variables. This is discussed further in this chapter. 

In the previous chapter we discussed response bias, a common threat to the psychometric quality of tests. In this chapter we discuss the second major threat: test bias. Test bias arises when the true scores and the observed scores differ between two groups. Think of men and women as two groups. The emphasis of test bias is therefore on systematic differences between groups of respondents. Please note that the identification of differences between groups does not necessarily mean that there is also (test) bias; it may be that these differences are actually present in reality that way. 

What types of test bias are there?

There are generally two types of test to distinguish bias: construct predictive bias and bias. Construct bias: bias regarding the meaning of a test. Predictive bias: bias regarding the usability of a test. These two types of test bias are independent of each other. In other words, one bias can exist in a certain test without the other bias.

1. Construct bias

Construct bias concerns the relationship between true and observed scores. This means that the test contains different interpretations of meaning from the two groups. If the interpretation differs per group, a test construct bias is created. This leads to situations in which two groups have the same average true scores but have different average observed scores in a test. 

2. Predictive bias

A predictive bias is a relationship between scores from two different tests. When one test (the so-called predictor test) contains scores that are used as a predictor for the scores of the other test (the so-called outcome test). A predictive bias exists when the relationship between the predictor test (true scores) and the outcome test (observed scores) differs between two groups. In other words, for one group the predictor test is a good predictor but for the other group the predictor test is a bad predictor.

What are the ways to identify test bias ?  

There are roughly two categories of procedures to identify test bias: (1) internal methods that identify construct bias; (2) external methods that identify predictive bias.

Although there is a difference in test scores between two groups, this does not necessarily mean that there is a test bias. Perhaps the difference is based on reality. For example: if a test shows that the weight of men is on average higher than the weight of women, then this is based on reality. But you can have your doubts when it comes to math skills. For example, it is not logical that the math skills of men are better than the math skills of women.

How can you discover construct bias?

Since we never know the true scores of a person, we use procedures that provide an estimate of the existence and extent of a construct bias.

We use internal structures to find out whether there is a construct bias. These contain a pattern of correlations between items and/or correlations between each item and the total score. Evaluation is as follows: we compare the internal structures for a test separately for two groups. If the two groups exhibit the same internal structures in terms of their test responses, we can conclude that the test does not suffer from construct bias. Conversely, if the two groups do differ in internal structures with regard to the test reactions, then there is construct bias.

There are five methods to discover construct bias:

1. Reliability.
2. Ranking (rank order).
3. Item discrimination index.
4. Factor analysis.
5. Differential item function analysis.

These five methods are discussed in more detail below.

1. Reliability

An intuitive way to evaluate the construct bias is by testing the reliability for each group separately. One of the ways to estimate reliability is through the internal consistent ( alpha coefficient ). In Chapter 6 we discussed that internal consistent refers to the degree to which parts of a test are interrelated. Translated into this context, this means that the alpha provides insight into the internal structure of a test, that is, are the test items consistent with each other or not. Group differences in reliability are an indication that the test does not "work" equally well for different groups.

2. Ranking (rank order)

If the items can be ordered by difficulty, we can use this ranking to make a relatively quick and easy estimate of the construct bias. This ranking can then be made between different groups, and then compare them with each other. If there is a difference in ranking of the items between the groups, there is an indication of construct bias.

3. Item discrimination index

The item discrimination index is a representation of the extent to which the item is related to the total test score. Item discrimination index distinguishes the variety in levels of the construct that is measured between people.  

An item makes a strong distinction between people with varying levels of the construct being measured, when people with a high capacity have a high chance of correctly answering the relevant question, which concerns the same capacity. However, people with a low capacity have a small chance of correctly answering the question in question, which is about the same capacity (= high item discrimination index value, eg 0.90). This means that the item is a good reflection of the construct that is measured by the test.

An item does not make a good distinction between people with varying levels of the construct being measured, when people with a low capacity give almost as many correct answers as people with a high capacity. An example of a low item discrimination index value is 0.10.

The item discrimination index can be used to estimate construct bias. An item is selected, from which we calculate the item discrimination index separately for each group. We then compare the group indexes per item. Equal indexes = no test bias. Uneven indexes = probably test bias. It is important to know that the item discrimination index is independent of the number of people in a group.

4. Factor analysis

A fourth way to investigate construct bias is through the use of factor analysis for items, where two or more groups are distinguished. As we discussed earlier, factor analysis is a statistical technique to divide the variance or covariance between test items into clusters or factors. A factor is a set of items that correlate highly with each other and therefore are interrelated. If all items correlate equally with each other, then there is one factor and we speak of a unidimensional structure. When there are several factors, we speak of a multidimensional structure.      

There are two trends in factor analysis: exploratory (exploratory factor analysis: EFA) and confirmatory ( confirmatory factor analysis: CFA). The latter, CFA in particular, is suitable for mapping construct bias. CFA is discussed further in the next chapter (chapter 12).

5. Differential item function analysis

Differential item function analysis gives the possibility to estimate the respondent's characteristic levels directly from test data scores. These are also called the true scores. We then compare the characteristic levels (true scores) with the item responses (observed scores) for all people in the two groups, and see if they match. If not, the item will suffer from bias.

• However, construct bias: two people (male and female) have the same characteristic level, but the item characteristic curve (ICC) is not the same. In other words, the chance that the two people give a correct answer is not the same as each other.
• No construct bias: two people (male and female) have the same characteristic level and the item characteristic curve is the same. In other words, the chance that the two people give a correct answer is equal.
• Uniform bias: differences in group in terms of curve location. The two lines do not overlap or cross each other. People from one group with the same characteristic level as people from the other group are less likely to answer the question correctly.
• Non-uniform bias: difference in group in terms of location and shape. The two lines overlap / intersect. At some levels the item is easier for men and at some levels the item is easier for women.

With uniform and non-uniform bias, the test measures different characteristics for men and women.

What are the ways to identify predictive bias? 

Predictive bias refers to the extent to which test cores are equally predictive of different groups. Ideally, when a test is used to make predictions about people, a test is equally predictive for all groups of people. If this is not the case, and the test is therefore not as predictive for different groups of people, then there is a predictive bias.  

Scores of two variables / measurements are obtained. Next, it is examined to what extent the scores of the first test can be used to predict the scores of the second test (which is related to the scores of the first test). An external evaluation of the test is required to discover the predictive bias. Two considerations are: (1) Does the test really help you predict the outcome? (2) Does the test predict the outcome evenly for several groups? We can investigate this on the basis of regression analysis. 

Regression analysis

Regression analysis contains linear relationships between test scores (true scores) and outcome scores (observed scores).

Ŷ = a + b (X), where X indicates the capacity score

• a = intercept (starts with X = 0);
• b = the direction coefficient;
• Ŷ = predicted value for individual.

The observed scores never exactly end up on the linear regression line. The regression line is formed from predicted scores, and the observed scores do not always match exactly.

''One size fits all'': The regression comparison is applicable to all groups. Different groups share a corresponding regression line, apart from gender, ethnicity, culture, or other group differences.

You can investigate whether the test contains bias by making a regression formula on the basis of data (for example, of both men and women). This is called the common regression line . We must create a regression line for each group separately (i.e, for men and women) and compare it with the common regression line. If these are not the same, then there is a predictive bias. If these are the same, then there is no question of a bias.

Within this method there are different types of bias: intercept bias, slope bias, and intercept + slope bias.

1. Intercept bias

With intercept bias, the direction coefficient of the two group regression analyzes corresponds to the common direction coefficient, but the intercept of the two group regression analyzes does not correspond to the common intercept.

One size does not fit all, so a predictive bias. In other words, there are different observed scores for men and women. Difference consistency exists because the difference between men and women remains the same as the X rises / falls. The two regression lines (of the men and of the women) are parallel to each other.

2. Slope bias

The intercept of the two group regression analyzes is the same as the common intercept . But the direction coefficient of the two group regression analyzes is not the same as the common direction coefficient.

'One size does not fit all ', so a predictive bias. In other words, there are also different observed scores for men and women here. There is no difference consistency, because the difference between men and women changes each time the X rises / falls. The regression lines (of men and women) do not cross each other.

3. Intercept and slope bias

The intercept of the groups is not equal to the common intercept and the direction coefficient of the groups is not equal to the common direction coefficient. This is much more common than that one part contains a bias and the other part does not contain a bias. Here, of course, there is also 'one size does not fit all '. The regression lines (of the men and of the women) do indeed intersect.

What other statistical procedures are there to detect test bias?

In addition to the methods we discussed in this chapter, there are a number of other statistical procedures to discover test bias. Structural equation modeling, for example, can be used to discover test bias. More complex regression models can also be used. But these methods (and their complexity) go beyond the scope of this book and are therefore not discussed further.         

What is the difference between test bias and test fairness?

Test bias is not the same as test fairness. Test fairness has to do with an appropriate use of test scores, in the field of social and / or legal rules and the like. Test fairness is not a psychometric aspect of a test. Test bias, on the other hand, is a psychometric concept, embedded in theory about test score validity. Test bias is defined by specific statistical and research methods, which enable the researcher to make decisions about the test bias. Both are important for psychological testing.     

In the previous chapter we discussed response bias, a common threat to the psychometric quality of tests. In this chapter we discuss the second major threat: test bias. Test bias arises when the true scores and the observed scores differ between two groups. Think of men and women as two groups. The emphasis of test bias is therefore on systematic differences between groups of respondents. Please note that the identification of differences between groups does not necessarily mean that there is also (test) bias; it may be that these differences are actually present in reality that way. 

The Generalizability Theory (G theory) helps us to distinguish the effects of multiple facets and then to use different measurement strategies. It is an ideal framework for complex measurement strategies in which several facets influence the measurement quality. This is a fundamental difference compared to the classical test theory (CTT), where different facets are not assumed.

What is a facet and what role do facets play in the complexity of a measurement strategy?

According to the G theory, measurement errors can be differentiated in different facets.

G theory can be used to investigate what effect the various aspects of a measurement strategy have on the total quality of the measurement. In this way the various items can also be examined. For example, when investigating which items are related to the onset of aggression each combination of items can be investigated separately. Each part of the measurement strategy is called a facet and different measurement strategies are partly defined by the number of facets. The more facets a measurement strategy has, the more complex the strategy is. An example of three facets is: items, observers and situations.

Which two key components play a role in G theory?

The concept of generalizability, as the name suggests, is very important within the G theory. The measurement quality is usually evaluated in terms of the ability to draw conclusions from a limited number of observations to an unlimited number of observations. When a psychological or behavioral variable is observed, only a limited number of observations can be made. The aim of the G theory is to obtain scores that are representative of the scores that would have been obtained if all possible items that could measure the construct were used.

The concept of consistency is also very important within the G theory. It is important to see whether the degree of variability of an individual's test scores is consistent with the variability of universal scores. In the G theory, estimates of generalizability are based on variance components which represent the extent to which differences exist within the 'universe' for each element of the design. A variance component is the variance of universal scores within the population of individuals. The magnitude of the variance component of a facet indicates the extent to which the facet influences observed scores.

Which two phases are distinguished in the G theory?

The G theory can be used for multiple types of analysis, but a basic psychometric analysis consists of a two-phase process: the G study and the D study. The variance components are estimated in the first phase. In such a study, factors are identified that influence the observed variance (and therefore the generalizability). This phase is called a G study, because it is used to identify to what extent the different facets could influence generalizability. In the second phase, the results of phase one are used to estimate the generalizability of the different combinations of facets. This phase is known as a D study, because the phase is used to make decisions about future measurement strategies.

The various steps associated with the two studies are discussed in more detail below.

1. G study

In this phase, variance analysis (ANOVA) is used to generate estimates of variance components for each factor. The purpose of ANOVA is to investigate the variability of a score distribution and to see the extent to which this variability is associated with other factors.

In a design with one facet there are three factors that can influence variability.

1. The extent to which the targets differ.
2. The extent to which the items differ.
3. Measurement errors.

For these three factors there are different formulas to calculate the variance components (see table 13.3 in the book).

In a design with one facet, the ANOVA gives two main effects and a residue (error).

The result that we are most interested in is the target effect . This reflects the extent to which targets have different averages. The target effect is the 'signal' that a researcher is trying to discover. In a design with one facet, the residual effect is the 'noise' that potentially marks the signal of the target effect. If the measurement is good, participants who score high on one item will do the same on the other items. When the items are inconsistent it indicates that there are no clear differences between the individuals and that the items may not be good reflections from the construct.

2. D study

During this phase, the psychometric quality of different measurement strategies is estimated that can help in planning a good measurement strategy for the research in question. In this phase, coefficients or generalizability for different measurement strategies are estimated. These coefficients vary between 0 and 1.0. The following applies:

Generalizability coefficient = Signal / ( Signal + Noise )

For the formulas and one worked out to calculate the relative generalizability of the differences between targets, see book page 435-436. 

There is an important difference between a design with one facet and a design with multiple facets. This difference lies in the complexity of the components that influence the variability in the data. When a new facet is added, new components are also added. This complexity makes the ' noise ' or 'error element' of the generalizable coefficients more complex.

Examples of 'one-facet designs' and 'multiple facet designs' can be seen in the book.

Which other measurement designs are there? 

There are at least four important ways in which a G theory analysis can differ compared to another G analysis:

1. The number of facets;
2. random vs. fixed facets;
3. crossed vs. nested designs;
4. relative vs. absolute decisions.

1. The number of facets

The more facets there are, the larger and more complex the design and the more effects there are that generate variance components. The basic logic and process of the G theory is however the same with designs with fewer facets.

2. Random vs. fixed facets

If there is a random facet, then the items of the facet are chosen randomly from a sample of a universal number of items.

If there is a fixed facet, then all conditions of the facet are included in the analysis. People do not want to generalize outside the conditions used in the analysis .

The difference between using random or fixed facets can have important psychometric consequences. It can influence the psychometric quality of the research. It can also have consequences for the generalizability of the quality of the measurements.

3. Crossed vs. nested designs

In a multi-faceted analysis, the pairs of facets are crossed or nested. When a pair is crossed, all possible combinations of two facets are included in the analysis. If not all possible combinations are included, then it is a nested design. Determining this is important because it determines which effects can be estimated in a G analysis.

4. Relative vs. absolute decisions

A G theory can be used to make two types of decisions. Relative decisions contain the relative order of participants. When tests are used to make relative decisions, they are often referred to as norm- referenced tests . Absolute decisions are based on the absolute level of the score of an individual. When tests make such decisions, they are called criterion-referenced tests . Determining the difference between these two decisions is important because it influences the way noise or error is perceived. It influences the number of variance components that contribute to error when generalizibility coefficients are calculated. In most studies, researchers are more interested in the relative perspective than the absolute perspective. They are more interested in understanding relative differences between participants' scores on a measurement. So why some people score relatively high and others relatively low.

The Generalizability Theory (G theory) helps us to distinguish the effects of multiple facets and then to use different measurement strategies. It is an ideal framework for complex measurement strategies in which several facets influence the measurement quality. This is a fundamental difference compared to the classical test theory (CTT), where different facets are not assumed.