At the end of the nineteenth century, researchers became interested in mapping out the individual differences in people when it comes to their intellectual functioning. This also came with a new interest in experimental tradition.

What are the theories?

The first theory about intelligence came from Spearman who worked for Binet. He said that there was something what he called general intelligence (and he called this the g factor). This explains what a person's performance is on intellectual tests, known as IQ tests. Central to this theory is that intelligence is a hypothetical construct and out of this theory came the two-factor model of Herzberg.

A problem with theories about intelligence is that the evidence in the theories are mostly based on the subject groups. In factor analysis to do with intelligence researchers will make a distinction between 'scholastic' skills and the ability to solve new problems. Cattell also took Spearman's g factor and made a similar distinction between fluid intelligence (Gf); it is the ability to solve novel reasoning problems and is correlated with a number of important skills such as comprehension, problem solving, and learning. On the other hand there is crystallized intelligence (Gc); it is the ability to deduce secondary relational abstractions by applying primary relational abstractions to each other, but to be able to deduce you have to use fluid intelligence.

The most common used method for determining a person's intelligence has to do with the use of an intelligence test. These tests usually consist of a number of subtests, which assess specific intellectual skills.

What is the Wechsler Adult Intelligence Scale-IV?

The WAIS-IV-NL consists of fifteen subtests that cover different cognitive abilities. The raw scores on the subtests are converted to scaled scores on the basis of age-related norm groups. The scaled scores of the subtest are used to calculate the IQ score. The subtests are: vocabulary, similarities, information, understanding, series of numbers, arithmetic, checking numbers and letters, block patterns, and matrix reasoning.

What is the Groninger Intelligence Test-2?

The GIT is an individual intelligence test that was developed in the Netherlands in the late 1950s. The objective was to construct a test that would be applicable to a wide age range in which a number of intelligence factors, which were commonly known at that time, would be represented. You have the subtests: Woordenlijsten, Legkaarten, Vaaropdrachten, Sorteren, Figuur ontdekken, Cijferen, Draaikaarten, Matrijzen, and Woord opnoemen.

What is the Kaufman Adolescent and Adult Intelligence Test?

The KAIT is an instrument for determining intelligence, intended for people between the ages of 14-85. The core battery consists of six subtests, three for measuring fluid intelligence and three for measuring crystallized intelligence, which can be administered in approximately 60 minutes. The fluid IQ represents a person's intelligence level when he is faced with unresolved problems, and the crystallized IQ gives an indication of mental functioning in the case of problems that are easier to solve as a result of more experience and training. 

What is the Raven's progressive matrices?

Raven's progressive matrices is a non-verbal test that measures visual problem solving and the test can be used as a measure of fluid intelligence. There are three common forms of this test: the standard progressive matrices, the colored progressive matrices and the advanced standard progressive matrices versions. The multiple-choice task involves selecting the piece with the pattern that is best suited to the rest of the pattern. 

What is the Dutch reading test for adolocents?

The NLV was originally developed for obtaining a reliable estimate of the level of premorbid intellectual functioning in patients with cerebral dysfunctions within a short time frame. The basic assumption is that a person's vocabulary, as a part of the crystallized intelligence, is resistant to cognitive decline, so that's also the part that you can check with the NLV.  

How do intelligence tests work in actual practice?

The tests described are all used to get a IQ score, but this is an estimate. Since the introduction of the IQ as a number, a lot of attempts to have been made to create a uniform classification system, often with the objective of a particular selection. The problem that arises here is that the reliability/unreliability of the IQ test normally fails to be taken into account and the IQ is often reported as an 'absolute number', instead of being reported as a value with a corresponding confidence interval.

Another problem with classifying individuals is the way in which the test score is achieved. An IQ value should always be mentioned together with the corresponding confidence interval, but using different intelligence tests may also result in different IQ scores, and into different descriptors.

What has intelligence to do with level of education?

You also have to think about the question how you can measure someone's intellectual functioning exactly and objective. Someone's IQ score can be a good prediction of school success or learning aptitude. But there are of course multiple types of intelligence, such as practical, emotional and creative skills that are not taken into account by a simple IQ test. 

Research has shown that there is a strong correlation between intelligence and school performance in tests where the emphasis is on reasoning and not on learning performance. But research also shows that general intelligence tests are not a good selection tool when you want to predict someone's success in a certain job. 

You can also use an Intelligence test in means of a neuropsychological test battery. When you use it as a neuropsychological test you can see whether the performance of someone on a subtest has become worse in relation to its premorbid level. There are certain subtests in the WAIS-IV, GIT and KAIT that can be used to diagnose possible neuropsychological disorders.