The sample comprised 650 females and 373 males who ranged in age from 54 to 94 years at baseline (mean=68.9 years; SD=6.9). Participants reported attaining 6 to 20 years of education (mean=14.1 years; SD=3.1). There was a significant negative correlation between age and educational attainment (r=−.117; p<.001). Compared to women, men reported significantly higher levels of education (t(683.420)=−3.343; p=.001), on average. There was no difference in age at study entry between men and women.
Unconditional growth models
Unconditional models for each composite containing only linear slopes were compared to corresponding models with both linear and quadratic slopes. For verbal fluency, verbal episodic memory and working memory, models that included both fixed and random effects of a quadratic slope did not converge. Thus, the model used in nested model comparisons for these three domains included only quadratic fixed effects. Adding quadratic change failed to improve model fit for any of these three cognitive domains. Thus, subsequent models included only linear slopes for verbal fluency, verbal episodic memory, and working memory. For processing speed, including a quadratic slope improved model fit (Δχ2(4)=−43.884, p<.001).
As shown in , significant linear decline over time was evident for verbal fluency, verbal episodic memory, and working memory. Comparing across domains, scores declined about 2–4% of one standard deviation (SD) per year. The fastest decline (3.5% SD per year) occurred in the working memory domain, and the slowest decline (1.5% SD per year) occurred in the verbal fluency domain. Significant positive quadratic (U-shaped) change was evident for processing speed, indicating slight improvements in speed (lower scores) between the first and second assessment waves, but slowing of performance (higher scores) thereafter. There was significant individual variation in cognitive ability in all four domains at baseline. Random effects in slopes were significant for working memory (p=.011) and verbal episodic memory (p<.001). Intercepts and slopes were not significantly correlated within any domain.
Fixed effects from the four separate unconditional growth models
Effects of age, gender, and education on cognitive performance and decline
Next, the three independent variables were added to the best-fitting models described above. The conditional model for processing speed would not converge without constraining the random variance of the quadratic slope to 0. In order to obtain estimates of the effects of interest (i.e., regression paths between the independent variables and cognitive change), the quadratic slope was removed from the model. It should be noted that the fixed effect of the linear slope in the unconditional model that did not include the quadratic slope was positive and significant (0.029; p<.001), which reflects increasing scores (slowing) of approximately 3% SD per year.
Older age at baseline was associated with worse cognitive performance in all four domains (). Age appeared to exert the greatest effect on processing speed, as each year of age greater than 70 reduced performance by nearly 5% SD. Age appeared to exert the smallest effect on verbal fluency, as each year of age greater than 70 reduced performance by only 2.3% SD. Older age was also associated with accelerated decline in all four domains.
Covariate effects in the four separate conditional models
Controlling for baseline age, gender was not associated with cognitive performance in processing speed, working memory, or verbal fluency. Gender was associated with poorer verbal episodic memory such that males performed approximately 1/3 SD worse on this composite, as compared to females. Gender was unrelated to the rate of decline in any domain.
Controlling for baseline age and gender, higher education was associated with better performance in all four cognitive domains. This beneficial effect of education appeared to be greatest in the verbal fluency domain, for which each year of education was associated with higher scores of nearly 11% SD. The influence of education on cognitive performance was smallest for the processing speed domain, for which each year of education was associated with higher scores of only 3.7% SD. Education was unrelated to the rate of decline in any domain, which can be visualized as parallel model-predicted trajectories in . The pattern of results did not change when the education variable was dichotomized at the sample median or split into tertiles ().
Effects of education (or literacy) on linear slopes in the supplementary models
Because younger individuals in the present sample reported higher levels of education than did older individuals, we examined the possibility that cohort effects masked association between education and cognitive decline. First, we ran all four conditional models excluding the covariate of baseline age. We also ran these models separately in subgroups of younger (<70 years) and older (≥70 years) adults. In all cases, we failed to find an association between education and change on any of the cognitive composites (). We also explored the potential for an alternative indicator of cognitive reserve (literacy) to moderate cognitive decline. We ran the four conditional models using scores on the vocabulary test at baseline in place of the education variable and found no differences in the pattern of results. That is, baseline vocabulary was positively associated with intercepts in all four conditional models, but unrelated to slopes.