Within-Person Distributions of Neuropsychological Test Scores as a Function of Dementia Severity

Objective: The distribution of a person's scores on a battery of neuropsychological tests reflects the nature and extent of intraindividual variability and may provide clinically useful information not captured by examining test scores in isolation. We sought to test the hypothesis that systematic alterations of within-person test-score distributions characterize worsening cognitive impairment. Method: We analyzed cross-sectional data from 2 datasets that included 395 clinical patients and 135 neurologically normal older adults (>= 60 years old). We computed each person's mean, standard deviation, skewness, and kurtosis for a battery of 13 neuropsychological measures and compared these distributional properties across groups. Results: Most healthy older adults produced normal (Gaussian) test-score distributions. Among patients, test-score distributions increasingly shifted away from normal with worsening cognitive impairment. Worsening dementia was accompanied by progressively lower mean scores and increasingly positive skew. Within-person standard deviations initially grew at mild levels of impairment, but then shrank with worsening dementia, resulting in positive kurtosis. Conclusions: Within-person distributional properties vary as a function of dementia severity. Despite the limitations associated with using a clinical rather than a research sample, the analyses reported here serve as a "proof of concept" and suggest that similar investigations with more rigorously characterized patient samples may be fruitful.