It is defined as the the ratio of the odds of an event occurring in one group to the odds of it occurring in another group, or to a data-based estimate of that ratio. These groups might be men and women, an experimental group and a control group, or any other dichotomous classification.

The logarithm of the odds-ratio is the difference of the logits of the probabilities.

The increased use of logistic regression in medical and social science research means that the *odds-ratio* is commonly used as a means of expressing the results in some forms of clinical trials, such as case-controlled trials, and in survey research.