Within-Person Test Score Distributions: How Typical Is “Normal”?

Abstract

We evaluated within-person variability across a cognitive test battery by analyzing the shape of the distribution of each individual’s scores within a battery of tests. We hypothesized that most healthy adults would produce test scores that are normally distributed around their own personal battery-wide, within-person (wp) mean. Using cross-sectional data from 327 neurologically healthy adults, we computed each person’s mean, standard deviation, skew, and kurtosis for 30 neuropsychological measures. Raw scores were converted to T-scores using three degrees of calibration: (a) none, (b) age, and (c) age, sex, race, education, and estimated premorbid IQ. Regardless of calibration, no participant showed abnormal within-person skew ( wp skew) and only 10 (3.1%) to 16 (4.9%) showed wp kurtosis greater than 2. If replicated in other samples and measures, these findings could illuminate how healthy individuals are endowed with different cognitive abilities and provide the foundation for a new method of inference in clinical neuropsychology.

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