[영문]Recent studies have documented that normal adults exhibit considerable variability in cognitive performance from one occasion to another. We investigated this phenomenon in a study in which 143 adults ranging from 18 to 97 years of age performed different versions of 13 cognitive tests in three separate sessions. Substantial within-person variability was apparent across 13 different cognitive variables, and there were also large individual differences in the magnitude of within-person variability. Because people differ in the amount of short-term variability, we propose that this variability might provide a meaningful basis for calibrating change in longitudinal research. Correlations among the measures of within-person variability were very low, even after we adjusted for reliability, and there was little evidence that increased age was associated with a larger amount of within-person variability.