Cognitive Scores Vary As Much Within Test Takers As Between Age Groups
Giving a test only once isn't enough to get a clear picture of someone's mental functioning.
It appears that repeating tests over a short period may give a more accurate range of scores, improving diagnostic workups.
How precise are tests used to diagnose learning disability, progressive brain disease or impairment from head injury?
Salthouse gave 16 common cognitive and neuropsychological tests to evenly divided participants (90 in the first, 1600 in the second) into groups of ages 18-39, 50-59 and 60-97 years old. In both studies, the variation between someone's scores on the same test given three times over two weeks was as big as the variation between the scores of people in different age groups. It's as if on the same test, someone acted like a 20-year-old on a Monday, a 45-year-old the following Friday, and a 32-year-old the following Wednesday. This major inconsistency raises questions about the worth of single, one-time test scores.