Psychological Tests and Intellectual Disabilities
Tests of intellectual functioning (IQ)
Tests of intellectual functioning are designed to measure different mental abilities. These tests are commonly called IQ tests. IQ tests measure the following mental abilities:
- problem solving;
- abstract thinking;
- academic learning;
- experiential learning
Previously an IQ score of 70 or below was the recommended cutoff score. The DSM-5 (APA, 2013) has de-emphasized specific IQ scores. Nonetheless, an assessment of intellectual functioning remains central to diagnosis. Thus, IQ scores are still very important considerations. Therefore, it's useful to understand what IQ scores mean.
If everyone took an IQ test there would be a wide variation in scores. This is called a distribution of scores. The average score (the mean) is the score that is the smallest distance away from all the other scores in the distribution.
When naturally occurring attributes such as IQ (or hat size, for that matter) are measured, the distribution looks like inverted 'U' shape curve. This curve is called the normal curve. Normal curves have special properties. The area under the normal curve represents the chances of observing a certain score. Then we know how common or uncommon a certain score is.
IQ test scores are standardized. Standardized tests allow a person's score to be compared to other people. IQ tests have a mean score of 100 and a standard deviation of 15. Standard deviation can be understood as the shape of a curve. Some U shaped curves are low, flat, and wide. Other U shaped curves are high, tight, and narrow. Mathematically, standard deviation is a specific distance from the mean. Standard deviation describes this distance and thus the shape of the curve.
Because of mathematical qualities of normal curves, we know that 68% of the area under the normal curve is one standard deviation above and below the mean. Therefore, IQ scores falling between 85 and 115 are common. About 68% of the population would score in that range. About 95% percent of the area under the normal curve occurs between two standard deviations. Therefore, IQ scores falling between 70 and 130 occur 95% of the time. There is a remaining 5%. Half or 2.5% will be above a score of 130. The other 2.5% will be below a score of 70. Therefore, we would expect that only 2.5% of the population would score above 130. This group represents people with an unusually high IQ. They are often termed, "gifted." We would also expect that only 2.5% of the population would score below 70. Historically, scores below 70 indicated an intellectual disability.
IQ tests have two parts. One part measures verbal abilities. The other part measures spatial abilities. Spatial is sometimes called performance skills. It refers to movement and the manipulation of three-dimensional space. Verbal and spatial scores vary widely. One person might have low scores across the board. Another person might do well verbal IQ but poorly on spatial IQ. Therefore, even though two people have the same total IQ scores, their abilities may be very different.