Denote the observed data from a case-control study in the form
of a traditional epidemiological 2x2 table using: a,b,c,d. Denote the
analogous population contingencies as: A,B,C,D. Here, there are four
selection probabilities resulting from the four disease-exposure
contingencies: a/A, b/B, c/C, d/D. A quick way to evaluate the
impact of selection bias in a case-control study is to calculate
a "selection odds ratio": [(a/A)*(d/D)] / [(b/B)*(c/C)]. Multiplying
an observed odds ratio by the inverse of this selection odds ratio
will result in an adjusted estimate.
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