Cohort Studies

Cohort studies differ from case-control studies in that the cohort moves forward in step, whereas the case-control study is a retrospective, historical analysis of prevalent cases. Longitudinal study designs sometimes seek to detect and then adjust for contributions to measurement error made by previous exposure to the same measurement. This arises in many settings but is most problematic when mental performance is measured. It may surprise the investigator that even after intervals, each of several years, some old people— mainly the better educated—maintain consistent improvements, whereas the less able or educated may under-perform on first testing, improve on the first re-retesting, and thereafter show expected age-related decline. These problems may be detected in study designs where several consecutive cohorts (say, now aged 60-64; 65-69; 70-74 years, etc.) are recruited, and at each reassessment these cohorts are added to by subjects of the same age but who did not take part in the earlier assessment (Schaie, 1994). Obviously, these sampling processes are demanding of time and effort and should be envisaged only when the research questions demand it; most often simple methods can provide useful results commensurate with many issues involved. A key advantage of cohort studies is that these provide estimates of relative risk, whereas cross-sectional studies permit calculation of the odds ratio. In practice, the odds ratio provides an acceptable estimate of relative risk. There are many good accounts of cohort study design in nutritional epidemiology (e.g., Willett and Colditz, 1998; Day et al., 1999), and methods to combine data from several studies are also available (Mannisto et al., 2004).

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