Background: Analysis of change in health status using data from two waves can be examined either adjusted or unadjusted for baseline health status. We assess the effect of socioeconomic position (SEP) on cognitive change using both these strategies and discuss the implications of the analyses.
Methods: Data come from 1261 men and 483 women of the Whitehall II cohort study, aged 50-55 years at wave 1. Cognition was assessed at both waves using a test of verbal memory, and two tests of verbal fluency. Analysis of Variance (ANOVA) was used to estimate the effect of SEP on change score and analysis of covariance (ANCOVA) to estimate this effect adjusted for the baseline cognitive score. Then the ANCOVA estimates were corrected for bias due to measurement error (estimated based on 3-month test-retest). Finally, ANCOVA estimates were examined for increasing levels of measurement error.
Results: The results of the ANOVA suggest no effect of SEP on cognitive decline. In contrast, the ANCOVA suggests significantly greater cognitive decline in the lower SEP groups. However, the ANCOVA estimates for the effect of wave 1 cognition show evidence for regression to the mean due to the presence of measurement error. The corrected ANCOVA estimates show no association between SEP and cognitive decline.
Conclusions: We recommend caution when using ANCOVA, or adjustment for baseline, in the analysis of change using two waves of observational data.
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