Exposure opportunity in epidemiologic studies.

We have shown that consideration of exposure opportunity is crucial to planning epidemiologic investigations because it focuses studies on questions of relevance, it helps to identify links to variables that can distort the assessment of an exposure's effect, and the cost of investigation can be reduced by the use of appropriate eligibility criteria. The reasons given by Poole for ignoring exposure opportunity are based on misleading claims of increased precision, disregard for probable confounding by factors related to exposure opportunity, and failure to consider realistically the cost of obtaining data on persons whose experience is irrelevant to the issues under investigation.