Normal aging increases cognitive heterogeneity: analysis of dispersion in WAIS-III scores across age.

Individual differences in cognitive decline during normal aging need further delineation. The purpose of this study was to find the score dispersions in the WAIS-III subtests at different ages. Norms presented in the Administration and Scoring Manual [Wechsler, D. (1997). WAIS-III: Administration and scoring manual. San Antonio: The Psychological Corporation] were used. The WAIS-III was standardized and normalized using 2450 American adults divided into 13 age ranges and 4 education groups. Means and standard deviations for the different WAIS-III subtests were deduced and the ratio Percentage of the mean="(standard deviation/mean)x100" was calculated. It was hypothesized that during normal aging, whereas mean scores decrease, score dispersions increase, pointing to an increased heterogeneity in intellectual abilities in older individuals. In all subtests, except Digit Span, it was found that score dispersions indeed increased during aging. However, in some subtests, increase in dispersion was less than 20% (Block Design, Object Assembly, and Information), whereas in others, increase in dispersion was over 200% (Matrix Reasoning, L-N Sequencing, Digit-Symbol, Picture Completion, and Picture Arrangement). It was proposed that cognitive heterogeneity during normal aging is related to those abilities measured with these latter subtests, basically, executive functions, attention, and selected non-verbal abilities. In other abilities (e.g., visuoconstructive abilities and fund of general information), normal aging is associated with a more homogenous pattern of decline.