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 derailed 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.