Dispersion in cognitive ability as a function of age: A longitudinal study of an elderly community sample

This longitudinal study investigated whether age is associated with increased dispersion among major domains of cognitive ability. Three samples were examined: the full sample of 760 elderly community dwellers aged 70 years and older who were tested in 1990; a subset of the original sample who died between testing occasions; and the sample of 426 who survived with full data sets in 1994 (followed up for a mean 3.5 years). Dispersion, as measured by the within-individual standard deviation of ability scores and by the within-individual deviations from crystallized intelligence for speed, memory and spatial functioning, was significantly correlated with age in all three samples at Wave 1 and at Wave 2 (for the longitudinal sample). The rate at which dispersion increased was not significantly correlated with age. In a more detailed analysis of the 426 survivors, dispersion as a function of age was similar for demented persons within this sample, those without dementia, those with poor and excellent educational levels, and those with a physical disability. Activities of daily living was a predictor of larger-than-average changes in dispersion - but not age, education, or activity. Greater dispersion was associated with faster deterioration in memory and speed performance. Contrary to some recent reports, there was evidence for greater within-individual variability among cognitive domains in older individuals.