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:

• reasoning;
• problem solving;
• abstract thinking;
• judgment;
• 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