Correlates of within-person (across-occasion) variability in reaction time.

A total of 420 adults between 18 and 91 years of age carried palm pilot (Palm M100; PalmOne, Milpitas, CA) devices and performed several reaction time trials when prompted at random times during the day. On the average, within-person variability in median reaction time from one occasion to the next was nearly the same magnitude as the between-persons variability in across-occasion mean reaction time. Analyses controlling the variation in one variable when examining relations involving the other variable suggested that an individual's mean reaction time is a more powerful predictor of relations with age and with the level of various cognitive abilities than his or her across-occasion SD. 2005 APA