A total of 760 participants were enrolled in the study and had baseline cognitive testing. Of those, 688 individuals completed at least one follow-up evaluation. Forty-one of this 688 had missing data for at least one of the covariates in this study (age, education, gender, acculturation, language of administration), yielding a total sample of 639. Six follow-up evaluations were completed by 217 of the 639, 115 completed five follow-ups, 77 had four follow-ups, 86 had three, 72 had two follow-ups, and 72 had one follow-up. The average age of the sample at baseline was 70.1 (SD = 6.7). Individuals were allowed to decide which language they preferred for testing; 63% were tested in Spanish, the remainder in English. The mean acculturation level was 42 (SD = 12.7). Scores on this scale ranged from 7 (Mexican oriented) to 62 (Anglo-oriented), indicating a wide distribution of acculturation levels. The mean level of education for this sample was quite low (mean = 7.3, SD = 5.5), with more than 50% of the sample having less than a high school education. However, the education level spanned a very wide range from zero years of formal education to post-graduate studies. Mean baseline scores for the cognitive and functional predictor variables are included in . At baseline, 88% of the sample was cognitively normal and 12% had mild cognitive impairment or dementia.
Baseline cognitive and functional scores.
Demographic predictors of global cognition
First, we examined baseline demographic variables in association with baseline and change in global cognitive functioning, as measured by the 3MS. In a model that included all the demographic variables (age, education, gender, language of test administration, acculturation), older age (in years; coefficient (standard error) = -0.40 (0.06), z=-6.90, n=639, p <.001), being tested in English (-2.82 (1.21), z=-2.32, n=639, p=.02), and Anglo acculturation (-0.10 (0.05), z=-2.00, n=639, p = .05) were associated with poorerbaseline global cognitive function, while higher education (in years; 1.27 (.09), z=13.87, n=639, p<.001) was associated with better performance. When these same demographic variables were examined in relation to longitudinal change in global cognitive status, older age at baseline (-0.09 (.01), z=-7.67, n=639, p = <.001) was associated with a faster rate of decline on the 3MS over time and male gender (0.30 (.16), z=1.96, n=639, p = .05) was associated with slower decline.
Baseline neuropsychological predictors of global cognition
Next, baseline measures of specific neuropsychological tests were examined in association with baseline and change in global cognitive function. Baseline 3MS was positively related to baseline Verbal Memory (standard score units; 5.84 (0.44), z=13.27, n=639, p<.001) and Object Naming (standard score units; 6.88 (0.37), z=18.88, n=639, p<.001). Furthermore, baseline performance in Verbal Memory (0.36 (0.11), z=3.41, n=639, p=.001) and Object Naming (0.39 (0.09), z=4.55, n=639, p<.001) were independently associated with 3MS change, with lower cognitive scores predicting greater decline. When the demographic variables and the neuropsychological variables were included in a joint model, both neuropsychological variables remained associated with baseline global cognition (Verbal Memory (5.48 (0.46), z=11.92, n=639, p<.001), Object Naming (5.31 (0.46), z=11.48, n=639, p<.001)), along with years of education (0.72 (0.08), z=9.03, n=639, p<.001), and being tested in English (-3.15 (0.97), z=-3.24, n=639, p=.001). However, when predicting longitudinal change in global cognition, baseline Verbal Memory was no longer a significant predictor (0.20 (0.12), z=1.72, n=639, p = .09). Worse Object Naming remained significantly associated with faster decline in global cognition (0.24 (0.11), z=2.14, n=639, p = .03) along with older age (-0.08 (0.01), z=-6.11, n=639, p < .001); male gender (0.32 (0.16), z=2.01, n=639, p = .04) continued to be associated with improving performance.
Baseline functional status (IQCODE) as a predictor of global cognition
Degree of everyday functional impairment at baseline, measured by the IQCODE, was also examined in association with baseline and change in global cognitive status. Poorer baseline functional status was associated with poorer baseline 3MS (-10.98 (1.51), z=-7.30, n=639, p < .001). Greater baseline functional impairment was also strongly associated with a more rapid decline in global cognitive function over time (-1.36 (0.29), z=-4.65, n=639, p < .001). When demographic variables were added to the model, more impaired daily function on the IQCODE remained a strong predictor of longitudinal decline in global cognition (-0.98 (0.27), z=-3.58, n=639, p < .001), along with older age (-0.09 (0.01), z=-7.33, n=639, p < .001); being male was associated with less cognitive decline (0.33(0.15), z=2.18, n=639, p = .03).
Multivariate model: demographic, baseline cognitive and functional measures as predictors of global cognition
Finally, a joint model, including all of the demographic variables, the neuropsychological variables, and the IQCODE, was used to examine the independent predictors of baseline and change in global cognitive status. Continuous independent variables were standardized so that effect sizes of the various variables could be directly compared; the coefficients in this case show the change in 3MS baseline or change associated with a 1.0 s.d. difference in the independent variable. Lower education, being tested in English, and poorer performance on both Verbal Memory and Object Naming were all independently associated with poorer baseline global cognitive function (). Older age, being female, having more functional impairment (IQCODE) at baseline, and having poorer Object Naming were independently associated with greater longitudinal decline in global cognitive function over time (). The significant relationship between older age and progressive cognitive decline on the 3MS is illustrated in , which shows expected trajectories in rate of decline for three specific ages. models the predicted longitudinal trajectories in cognitive decline for three different IQCODE scores.
Independent effects of baseline demographic, neuropsychological, and functional predictors on estimated baseline global cognitive function
Baseline demographic, neuropsychological, and functional predictors of change in global cognitive function
Figure 2 Independent effects of age on 3MS baseline and linear rate of change. Results show model derived trajectories for three specific baseline ages (70, 75, and 80 years). The lines correspond to the expected average 3MS score at each time point. Bars show (more ...)
Figure 3 Independent effects of IQCODE on 3MS baseline and linear rate of change. Results show model derived trajectories for three specific baseline IQCODE scores (3.0, 3.5, and 4.0). The lines correspond to the expected average 3MS score at each time point. (more ...)