Background: Analysis of change in health status using data from two waves can be examined either adjusted or unadjusted for baseline health status. The effect of socioeconomic position (SEP) on cognitive change was assessed using both these strategies and the implications of the analyses are discussed.
Methods: Data from 1261 men and 483 women of the Whitehall II cohort study, aged 50–55 years at wave 1, were used. 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) was used to estimate this effect adjusted for the baseline cognitive score. 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: Results from analysis of change using two waves of observational data, when adjusted for baseline, should be interpreted with caution.
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Funding: AS-M is supported by a “European Young Investigator Award” from the European Science Foundation. MK and JV are supported by the Academy of Finland (project nos 117604, 124271, 124322 and 129264), MGM is supported by an MRC research professorship and MS is supported by the British Heart Foundation. The Whitehall II study has been supported by grants from the British Medical Research Council (MRC), the British Heart Foundation, the British Health and Safety Executive, the British Department of Health, the National Heart, Lung, and Blood Institute (grant HL36310), the National Institute on Aging (grant AG13196), the Agency for Health Care Policy and Research (grant HS06516) and the John D. and Catherine T. MacArthur Foundation Research Networks on Successful Midlife Development and Socioeconomic Status and Health.
Competing interests: None.