The IQ tests, such as the Raven’s Progressive Matrices or the Wechsler tests that include measures of verbal comprehension, perceptional reasoning, working memory, and processing speed, have been administered to large numbers of randomly selected people. From this sampling the distribution of scores is mapped onto a normal distribution with mean IQ equal to 100 and standard deviation 15. (WAIS-IV has 10 subtests and 5 supplemental tests that summarize intelligence with two measures–a final IQ score and a General Ability Index.) This process, called “norming the distribution” is a statistical process that is compliant with the scientific method and gives the percentile corresponding to each IQ score. For example, an IQ of 115 is one standard deviation above the norm and corresponds to the 84th percentile. This means that about 84% of the populations IQ scores are below 115.
However, many of the commercially available tests have not been normed on a large, statistically random sample and provide IQ measures that are not compliant with the scientific method.
