Cognitive Performance Inconsistency: Intraindividual Change and Variability Nilam Ram University of Virginia Patrick Rabbitt University of Oxford Brian Stollery University of Bristol John R. Nesselroade University of Virginia Although many studies have examined inconsistency of cognitive performance, few have examined how inconsistency changes over time. 91 older adults (age 52 to 79) were tested weekly for 36 consecutive weeks on a series of multitrial memory speed (i.e., letter recognition) tasks. A number of multivariate techniques were used to examine how individuals’ level of inconsistency changed across weeks and how this change was related to interindividual differences in age and intelligence. Results indicated that (a) inconsistency of performance is a construct separate from the underlying performance ability (i.e., memory speed); (b) inconsistency reduces exponentially with practice; (c) individuals with higher scores on tests of fluid general intelligence (G f ) reached lower asymptotic levels of inconsistency compared to lower scorers; and (d) after controlling for the systematic effects of practice, variability in inconsistency from week-to-week was more pronounced for individuals with lower G f scores compared to individuals with higher scores. Keywords: multivariate, memory speed, aging, growth curves, nonlinear Intraindividual variability in cognitive performance has some- times been conceptualized as inconsistency (e.g., Fuentes, Hunter, Strauss, & Hultsch, 2001; Hultsch, MacDonald, Hunter, Levy- Bencheton, & Strauss, 2000; Li & Lindenberger, 1999; Mac- Donald, Hultsch, & Dixon, 2003; Stuss, Pogue, Buckle, & Bondar, 1994; Stuss, Stethem, Hugenholtz, Picton, & Richard, 1989). De- fined as the variability of performance across occasions (e.g., Hultsch & MacDonald, 2004), inconsistency can be indexed or measured using the intraindividual standard deviation (ISD) of performance computed across occasions (Slifkin & Newell, 1998). Individuals whose performance varies little from occasion to oc- casion, regardless of level, are consistent (i.e., they have relatively low ISDs), whereas those individuals whose performance varies dramatically from occasion to occasion are inconsistent (i.e., they have relatively high ISDs). In this manner, intraindividual vari- ability across occasions can be indexed with a single score. This inconsistency score can then be examined in the same manner as any other intraindividual or interindividual difference variable. At a theoretical level, inconsistency of performance on relatively simple cognitive tasks has been considered a measure of variability in central nervous system functioning (Hendrickson, 1982; Hultsch & MacDonald, 2004; Li & Lindenberger, 1999). Consistent with this hypothesis, inconsistency (or intraindividual variability) in cognitive performance has been found to be related to age, injury, health, and intelligence (Hultsch & MacDonald, 2004). Generally, older adults tend to be more inconsistent in their performances than younger adults (e.g., Anstey, 1999; Fozard, Vercruyssen, Reynolds, Hancock, & Quilter, 1994; Hertzog, Dixon, & Hultsch, 1992; Hultsch, Mac- Donald, Hunter, Maitland, & Dixon, 2002; West, Murphy, Armilio, Craik, & Stuss, 2002), unhealthy or dysfunctional persons tend to be more inconsistent than healthy or functional persons (e.g., Fuentes et al., 2001; Spieler, Balota, & Faust, 1996; Stuss et al., 1994, 1989), and persons with lower levels of cognitive task performances (e.g., mea- sures of G f and G c ) tend to be more inconsistent than persons with higher levels of performance (e.g., Hultsch et al., 2002; Li, Aggen, Nesselroade, & Baltes, 2001; Rabbitt, Osman, Moore, & Stollery, 2001). In sum, on a variety of fronts, greater inconsistency seems to be a marker of impending decline or low functionality (Hendrickson, 1982; Li & Lindenberger, 1999; Rowe & Kahn, 1985, 1997). In most studies, inconsistency has been studied as the day-to- day or week-to-week variability in performance. However, incon- sistency can be measured across any time frame (Hultsch & MacDonald, 2004; Slifkin & Newell, 1998). For instance, an ISD calculated across multiple trials can be used to measure intraindi- vidual variability or inconsistency in the moment-to-moment fluc- tuations in performance over a short time period. Similarly an ISD calculated across observations gathered days or weeks apart can be used to measure inconsistency (or intraindividual variability) in performance over a longer time period. Nilam Ram and John R. Nesselroade, Department of Psychology, Uni- versity of Virginia; Patrick Rabbitt, Department of Experimental Psychol- ogy, University of Oxford; Brian Stollery, Department of Experimental Psychology, University of Bristol. Nilam Ram gratefully acknowledges the support provided by Grant T32 AG20500 from the National Institute on Aging in the preparation of this article. Special thanks to those at the Institute for Developmental and Health Research Methodology at the University of Virginia for helpful comments on earlier versions of this work. Correspondence concerning this article should be addressed to Nilam Ram, Department of Psychology, P.O. Box 400400, University of Virginia, Charlottesville, VA 22904-4400. E-mail: nilam@virginia.edu Psychology and Aging Copyright 2005 by the American Psychological Association 2005, Vol. 20, No. 4, 623– 633 0882-7974/05/$12.00 DOI: 10.1037/0882-7974.20.4.623 623