Selection bias has long been recognized as a potential threat to comparative studies. In the typhoid vaccine study over a century ago, the basic problems inherent in testing a new treatment on volunteers were well known. Yet, today there is still much confusion about the definition and implications of selection bias. Broadly speaking, selection bias can result from the manner in which individuals are chosen for inclusion in the study. However, there are at least three different aspects of a selection process that can result in bias. Discussions of selection bias often focus on one or another of these aspects. What appear to be general statements about selection bias may only be referring to a particular subcategory of selection bias. In this chapter, we attempt to unravel and clarify the various interpretations of selection bias. Our analysis will describe the issues from both the traditional statistical viewpoint and the emerging causal (counterfactual) perspective.
5.1 SELECTION PROCESSES AND BIAS
We define a selection process as the mechanism that determines which particular individuals from a target population become members of each particular study group (exposed and unexposed). In a widely referenced dictionary of epidemiology, Last (2001) defines selection bias as:
Error due to systematic differences in characteristics between those who take part in a study and those who do not.…Selection bias invalidates conclusions and generalizations that might ...