The odds ratio is a measure of
effect size particularly important in
Bayesian statistics and
logistic regression.It is defined as the ratio of the
odds of an event occurring in one group to the odds of it occurring in another group, or to a sample-based estimate of that ratio. These groups might be men and women, an experimental group and a
control group, or any other
dichotomous classification. If the probabilities of the event in each of the groups are p (first group) and q (second group), then the odds ratio is:
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The odds ratio is useful in the interpretation of the results of
Logistic regression (see Neter, Wasserman, and Kutner, 1989) and is computed from a 2x2 classification table which displays the predicted and observed classification of cases for a binary dependent variable:
(f11 * f22)/(f12 * f21)
where fij represents the respective frequencies in the 2x2 table.
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