Individual differences in decision speed have been regarded as direct reflections of a "primitive" functional neurophysiological characteristic, which affects performance on all cognitive tasks and so may be regarded as the "biological basis of intelligence", or of age-related changes in mental abilities. More detailed analyses show that variability within an experimental session (WSV) is a stable individual difference characteristic and that mean choice reaction times (CRTs) are gross summary statistics that reflect variability, rather than maximum speed of performance. A total of 98 people aged from 60 to 80 years completed 36 weekly sessions on six different letter categorization tasks. After effects of practice and of circadian variability had been eliminated, individuals with lower scores on the Cattell Culture Fair intelligence test had slower CRTs and greater WSV on all tasks. A simulation study showed that the greater WSVs of low Cattell scorers led directly to the significantly greater variability of their mean CRTs from session to session. However because CRTs on tasks co-varied from session to session it was apparent that, besides being affected by WSV, individuals' between-session variabilities (BSVs) also vary because of state changes that affect their performance from day to day. It seems that both variability in performance from trial to trial during a session and variability in average performance from day to day are correlated, stable, individual difference characteristics that vary inversely with intelligence test performance. Methodological consequences of these results for interpretations of age-related cognitive changes, for variability between as well as within individuals, for individual differences in decision speed, and for circadian variability in performance are discussed.

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-sub(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-sub(f) scores compared to individuals with higher scores.

Individual differences in decision speed have been regarded as direct reflections of a "primitive" functional neurophysiological characteristic, which affects performance on all cognitive tasks and so may be regarded as the "biological basis of intelligence", or of age-related changes in mental abilities. More detailed analyses show that variability within an experimental session (WSV) is a stable individual difference characteristic and that mean choice reaction times (CRTs) are gross summary statistics that reflect variability, rather than maximum speed of performance. A total of 98 people aged from 60 to 80 years completed 36 weekly sessions on six different letter categorization tasks. After effects of practice and of circadian variability had been eliminated, individuals with lower scores on the Cattell Culture Fair intelligence test had slower CRTs and greater WSV on all tasks. A simulation study showed that the greater WSVs of low Cattell scorers led directly to the significantly greater variability of their mean CRTs from session to session. However because CRTs on tasks co-varied from session to session it was apparent that, besides being affected by WSV, individuals' between-session variabilities (BSVs) also vary because of state changes that affect their performance from day to day. It seems that both variability in performance from trial to trial during a session and variability in average performance from day to day are correlated, stable, individual difference characteristics that vary inversely with intelligence test performance. Methodological consequences of these results for interpretations of age-related cognitive changes, for variability between as well as within individuals, for individual differences in decision speed, and for circadian variability in performance are discussed.

Study objectives-To examine the cognitive function in a large, ongoing cohort study of older men, and to identify associations with social and lifestyle factors. Design-A cross sectional study of cognitive function was conducted within the Caerphilly Prospective Study of Heart Disease and stroke. Setting-The Caerphilly Study was originally set up in 1979-83 when the men were 45-59 years of age. Extensive data are available on a wide range of lifestyle and other factors of possible relevance to cognitive decline. Associations between some of these and cognitive function are reported. Participants-A representative sample of 1870 men aged 55-69 years.Main results-Age, social class, medication, and mood were found to be powerful determinants of performance. Self report data on the involvement of the men in leisure pursuits were examined by factor analysis. This indicated that the more intellectual leisure pursuits are the most strongly linked with performance. A measure of social contact showed a weak positive association with the test scores. Current cigarette smokers gave lower test cognitive function scores than either men who had never smoked, or ex-smokers. There was however no evidence of any gradient in function with the total lifetime consumption of tobacco. The disparity between these two data sets suggests that there had been prior selection of men who had originally started to smoke, but more particularly selection of those who later quit smoking. There was no significant association between alcohol consumption and cognitive function, though ex-drinkers had markedly lower test scores than either current drinkers or men who had never drunk alcohol. This seemed probably to be a consequence of an high prevalence of illness among the ex-drinkers. Conclusions-Age and social class show strong associations with cognitive function. Leisure persuits and social contact are also both positively associated. Neither tobacco smoking nor the drinking of alcohol seem to be associated with cognitive function, though there is evidence suggestive of self selection of both men who had never smoked and ex-smokers.(J Epidemiol Community Health 1999;53:9-14)The rising numbers of older people in the community has led to an increasing concern with cognitive decline and dementia, and this has stimulated an increasing interest in aetiological factors other than age.