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.