Personality refers to a unique combination of psychological characteristics (e.g. cognitions, feelings, behaviours) that are relatively stable over time. A personality trait refers to any distinguishable, relatively enduring way in which one individual varies from another.

There are different trait theories and the different theories have different ideas of how many traits exist. The traits are relatively stable over time. However, the expression of a trait in behaviour can vary. The situation is important for the expression of a trait. Behaviour is trait and content-dependent (e.g. talking during a lecture is rude but talking at a café is friendly).

A personality state is a situation-specific, temporary disposition. A personality type is a constellation of personality traits. However, employing personality types might be a simplification of personality. A type-A personality is characterized by competitiveness (1), haste (2), restlessness (3), impatience (4), feelings of being time-pressured (5) and strong needs for achievement (6). A type B personality is characterized by the opposite of type A personality.

A personality profile refers to a narrative description, graph, table or other representation of the extent to which a person has demonstrated certain targeted characteristics (i.e. personality traits) as a result of the administration or application of tools of assessment.

The choice of assessment instruments depends on validity and reliability. In addition to that, it depends on the context and the research question.

High reliability is a condition for high validity. Norms of tests can be based on relative norm scores (i.e. how does one score compared to others) (1), relevance (2), representativeness (3), size (4) and actuality (5).

There are several types of instruments for assessing personality:

1. Projective tests
   These tests require a response to unstructured stimuli. The assessor draws inferences about personality based on the response of the test-taker. It makes use of association methods (1), constructive methods (2), completion methods (3), choice/ordering methods (4) and expressive methods (5).
2. Questionnaires
   These tests make use of self- or other-report. It employs different scales and there is a focus on the nomothetic approach in scoring and interpretation.
3. Observation
   Behavioural assessment can employ behavioural observation and rating scales (1), self-monitoring (2), analogue studies (3), situational performance measures (4), leaderless group technique (5), role play (6), psychophysical methods (7) or unobtrusive measures (8). It makes use of behaviour rating scales and the observation of behaviour can be direct or indirect and broad or narrow. It focuses on the idiographic approach.
4. Other methods
   The other methods to assess personality can include interviews or data analysis.

The scoring of projective tests is complex and is used for the recognition phase of the diagnostic process. Positive aspects of projective tests are that they are less reliant on reading skills (1), implicit processes (2), self-insight (3) and there is less faking (4). Negative aspects of projective tests are that they have low reliability (1), less standardization (2), poor validity (3) and impact situation (4).

Questionnaires are used for the recognition (1), explanation (2) and prediction (3) phase of the diagnostic process. Positive aspects of questionnaires are that there is standardization in administering and scoring (1), there are norms (2) and there is a lot of research on reliability and validity (3). Negative aspects of questionnaires are that it is very reliant on language skills (1), the norm group choice is arbitrary (2), there are different response styles (3) and there is response bias (4).

Observation is used for the recognition (1), treatment (2) and evaluation (3) phase of the diagnostic process. Positive aspects of observation are that it uses rating systems (1), it records behavioural strengths, weaknesses and patterns (2), there is less faking (3) and it is context-specific (4). Negative aspects of observation are that the costs are high (1), it takes a long time (2), it is unclear how it should be evaluated (3), there is reactivity (4) and there is observer bias (5).

The sign-approach states that test responses are deemed to be signs or clues to underlying personality or ability. The sample-approach states that behaviour is to be interpreted in light of a trait of interest. Frequency or event recording refers to recording each time behaviour occurs. Interval recording refers to recording behaviour according to predetermined schedules. Timeline follow-back (TLFB) methodology refers to a method of recording the frequency and intensity of target behaviour. Ecological momentary assessment refers to recording behaviour-related events as they occur through a handheld computer to maintain an electronic diary of the behaviour.

Behavioural assessment can provide data about the maintaining conditions of a certain behaviour and a baseline for studying an individual’s learning history. It can make use of composite judgement, the average of multiple judgements of multiple raters.

Behavioural assessment takes place to provide a behavioural baseline (1), provide a record of behavioural strengths and weaknesses (2), to pinpoint environmental conditions that maintain, trigger or extinguish behaviour (3), to target specific behavioural patterns for modification (4) and to create graphic displays useful in stimulating innovative or more effective treatment approaches (5).  

There are several problems with questionnaires when assessing personality:

1. Acquiescence
   This is the tendency to agree with questions. A solution to this is to use reversed questions.
2. Careless responding
   This is the tendency to answer questions randomly. A solution to this is to include an infrequency scale.
3. Extremity
   This is the tendency to give extreme answers to questions.
4. Fence-sitting
   This is the tendency to choose the middle option. A solution to this is to take away the middle option.
5. Social desirability
   This is the tendency to answer questions in a way that will make one look as socially desirable. A solution to this is to include a scale, using forced-choice or by preventing questions to allow for social desirability.
6. Faking good behaviour
   This is the tendency to deliberately distort answers to provide people with a positive image of oneself.
7. Faking bad behaviour
   This is the tendency to deliberately distort answers to provide people with a negative image of oneself.

Raters are prone to the leniency error (1), generosity error (2), severity error (3), halo effect (4), horn effect (5) and error of central tendency (6). The rater needs to take the context of the evaluation and the cultural background of the assessee into account.

A personality test consists of statements about a person (1), consists of several scales (2) and the scales consist of several items (3). There are several often-used personality tests:

1. Myers-Briggs Type Indicator (MBTI)
   This questionnaire is mainly used in context of training (1), team building (2) and coaching (3). It measures four traits. However, there is little scientific evidence for the theory it is based on (1), it has low reliability for the types (2) and there is poor validity (3).
2. Minnesota Multiphasic Personality Inventory 2 (MMPI-2)
   This questionnaire is mainly used to measure personality and psychopathology. However, it contains a large pool of poorly formulated questions (1) and there is a lot of overlap between the scales (2).
3. NEO-PI-R
   This questionnaire is used to measure personality according to the Big Five. However, there is a poor criterion validity for the five scales that are used (1) and the formulation of the questions is often complex (2).
4. Five-Factor Personality Inventory-II (FFPI-II)
   This questionnaire is used to measure personality according to the Big Five with one scale adjusted for the Dutch population. A problem is that the scores need to be calculated using a computer.
5. Dutch Personality Questionnaire (NPV)
   This questionnaire is used to measure a wide array of personality constructs that are relevant to different practice areas. This test is generally seen as a good test.

A self-concept measure refers to an instrument designed to yield information relevant to how an individual sees oneself concerning the selected psychological variables. Self-concept differentiation refers to the degree to which a person has different self-concepts in different roles.

An advantage of an a-theoretical personality test is that test users can impose their theoretical preferences on the interpretations of the findings. The frame of reference refers to the time frame and/or contextual issues that influence the focus of exploration (e.g. personality information about how a person wants to be). The Q-sort technique, which requires test-takers to sort a group of statements (e.g. ranking from least to most descriptive) is used to assess different frame of references.

There are several tools in the process of developing personality tests:

1. Logic and reason
   This can determine the content of the items of a test.
2. Theory
   This can alter the interpretations and construction of items in personality tests.
3. Data reduction methods
   This can alter the items and scales used in a personality test (e.g. factor analysis changes the scales that are used in a test).
4. Criterion groups
   This can serve as a standard according to which items will be included or discarded from the final version of a scale.

Criterion groups refer to groups of test-takers who share specific characteristics among each other and with the population of interest. Empirical criterion keying refers to the process of using criterion groups to develop test items. A criterion refers to a standard on which a judgement or decision can be made.

The development of a test employing empirical criterion keying has several steps:

1. Create a large, preliminary pool from which the test items for the final test will be selected.
2. Administer the pool to two groups of people; a criterion group (1) and a randomly selected group of people (2).
3. Conduct an item analysis to select items indicative of membership in the criterion group.
4. Obtain data on test performance from a standardization sample of test-takers who are representative of the population.

The ipsative approach is an approach in which a test-taker's responses and presumed strength of a measured trait are interpreted relative to the strength of the traits of the individual.

Personality assessment is useful for choosing a career (1), personnel selection (2) and clinical diagnostics (3). After intelligence, personality is the best predictor for work performance.