Implications of within-person variability in cognitive and neuropsychological functioning for the interpretation of change.

Samples of adults across a wide age range performed a battery of 16 cognitive tests in 3 sessions within an interval of approximately 2 weeks. Estimates of within-person variability across the 3 assessments were relatively large and were equivalent in magnitude to the cross-sectional age differences expected over an interval of 15-25 years. These findings raise questions about the precision of assessments based on a single measurement and imply that it may be difficult to distinguish true change from short-term fluctuation. Because there were large individual differences in the magnitude of this variability, it is proposed that change might be most meaningfully expressed in units of each individual's own across-session variability.