The ratio of the odds of the risk factor in a diseased group and in a non-diseased (control) group. The odds ratio is used in retrospective case-control studies (relative risk is the ratio of proportions in two groups which can be estimated in a prospective study). Odds ratio and relative risk are usually considered interchangeable especially for large samples and are calculated as ad/bc where a,b,c,d are the entries in a 2x2 contingency table (hence the alternative definition as the cross-product ratio). In logistic regression, the coefficient corresponds to the of the odds ratio. There are statistical methods to test the homogeneity of odds ratios. (Online Odds-Ratio Calculation.)