Odds ratio is a ratio that measures effect size, that is the strength of association between 2 dichotomous variables, one describing an exposure and one describing an outcome. It represents the odds that an outcome will occur given a particular exposure, compared to the odds of the outcome occurring in the absence of that exposure ( the probability of the event occuring divided by the probability of an event not occurring). The odds ratio is a ratio of describing the strength of association or non-independence between two binary data values by forming the ratio of the odds for the first group and the odds for the second group. Odds ratio are used when one wants to compare the odds of something occurring to two different groups.