Severity of Selection Bias in Cohort Studies
Date Issued
2012
Date
2012
Author(s)
Lin, Jia-Chun
Abstract
In cohort studies, researchers need to exclude prevalent cases, as well as cases in the preclinical period of the disease under study. However, these two kinds of cases are often difficult to be diagnosed, and therefor cannot be excluded from the study. This will create selection bias. In this study, we perform computer simulation to examine the magnitude of selection bias at each time point after the inception of a cohort study. We found that the direction of selection bias is toward the null. We also found that the selection bias can be ignored for rare diseases but not for common diseases (the bias is approximately for common diseases). Excluding the preclinical subjects at the inception of a cohort study will decrease the magnitude of the selection bias, but cannot completely eliminate it. In addition, we found that the selection bias needs a long follow-up time to dissipate. We encourage researchers to conduct active follow-up during the follow up, which can significantly reduce the follow-up time needed for dissipation of the selection bias. Active follow-up and excluding the data of the first year of follow-up can eliminate most selection bias of a cohort study.
Subjects
Selection bias
cohort study
common disease
active follow-up
toward the null
Type
thesis
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