A comparison of the probability of one outcome given a circumstance to that of a separate outcome given a separate circumstance. In a classical example, an odds ratio compares binary disease outcomes to binary exposures. To calculate an odds ratio one would use a 2×2 contingency table (see Table) summarising the frequency of observations for each group in the presence and absence of an exposure. The odds from Group 1 are a/b, and the odds for Group 2 are c/d. The odds ratio is a comparison of the two, which simplifies to ad/bc. An odds ratio of one implies that the event is equally likely for both groups; greater than one implies that the event is more likely in the first group; less than one implies higher likelihood in the second group.