Spearman coined general intelligence and stated that this explained a person’s performance on intellectual tests. However, it was contested that there is one factor explaining performance on intellectual tests and multiple factors were formulated.

There are scholastic skills and skills to be able to solve new problems. Crystallized intelligence refers to the knowledge of facts and fluid intelligence refers to the ability to solve new problems and reason. Crystallized intelligence is relatively stable whilst fluid intelligence is not and is disrupted by several factors (e.g. brain damage, age).

The general intelligence test is a method of assessing someone’s intelligence and consists of several sub-tests which assess specific intellectual skills. IQ reflects a person’s overall performance on all subtests in comparison with a group of peers.

The Wechsler Adult Intelligence Scale-IV (WAIS-IV-NL) is an intelligence test that consists of 15 subtests that cover working memory (1), arithmetic (2), vocabulary (3), information processing speed (4), power of abstraction (5) and visual spatial problem solving (6). It is possible to do a shorter version of this, although this is less generalizable than the original, 70-minute version.

The Groninger Intelligence Test-2 (GIT-2) is an individual intelligence test which is applicable to a wide age range whilst representing as many intelligence factors as possible.

The Kaufman Adolescent and Adult Intelligence Test (KAIT) is an individual intelligence test for determining intelligence for people between the age of 14 and 85. It can be administered in two ways. The core battery (i.e. 3 subtests for crystallized intelligence and 3 subtests for fluid intelligence) can be administered. A longer version can also be administered. The total IQ should not be interpreted when the IQ for the crystallized intelligence and the fluid intelligence differ a lot.

Raven’s progressive matrices is a non-verbal test that measures visual problem-solving and can be used as a measure of fluid intelligence. It is an analogue reasoning task. The forms of this test are the Standard Progressive Matrices (1), the Coloured Progressive Matrices (2) and the Advanced Standard Progressive Matrices (3). The advantages of using these tests are that it does not require a verbal response (1) and that it is mostly culture-free (2).

The Nederlandse Leestest voor Volwassenen (NLV) was developed for obtaining a reliable estimate of the level of premorbid intellectual functioning in patients with cerebral dysfunctions within a short time frame. There is a high correlation between the results on this test and verbal IQ.

A lot of consequences (e.g. special education or not) depend on the results on IQ scores and these consequences treat IQ scores as absolute numbers.

One problem with such classification system is that intelligence tests often do not differentiate properly between participants on the extreme scale of the measure. Another issue is the Flynn effect. It is unclear whether intelligence declines with old age or whether information processing speed declines.

An intelligence test can provide powerful diagnostic information. However, it might not be useful for a localized disorder or for localized damage. There are three levels in intelligence tests in neuropsychological questions:

1. Level of the test performance
   This involves the level of the performance on the intelligence test and is related to a person’s background. At this level, the intelligence test is used for its original purpose.
2. Inventory of disorders
   At this level, the intelligence test is not used for its original purpose but is used as a neuropsychological test battery.
3. Statements about abnormal performances on IQ tests
   These statements often include localized brain damage.