Odds ratio

An odds ratio (OR) is a measure of the proportional excess risk of an event in a population compared with the risk in another population. When the populations are defined by treatment choice (but otherwise identical, as in a clinical trial), this gives a measure of the relative effect of an intervention. The OR is the odds of an event occurring in the intervention group divided by the odds of the same event occurring in the comparison (control) group. The odds of an event are calculated as the number of subjects in a population experiencing an event divided by the number of subjects in the population not experiencing the event. An OR equal to 1 indicates that there is no difference between the groups (i.e. the event is equally likely to occur in the intervention group and control group). An OR greater than 1 indicates that the event is more likely to occur in the intervention group than in the control group. An OR less than 1 indicates that the event is less likely to occur in the intervention group than in the control group. Compared with relative risks, ORs have statistical properties that make them especially useful for meta-analyses. However, they may need to be translated to relative risks if differences in absolute effect are required, such as in cost-effectiveness analyses. In the case of rarer events (e.g. occurring to <20% of the population), ORs and corresponding relative risks tend to be similar in magnitude.