**The Relative Risk (RR)** is used to compare the probability of an event between two different groups. It is simply the ratio of the probability of the event occurring in two, mutually exclusive groups:

RR = π1 / π2

A RR of 1 means there is no difference in risk between the two groups. A RR of < 1 means the event is less likely to occur in π2. A RR of > 1 means the event is more likely to occur in π1

Example: You might calculate the relative risk of PONV between men and women.

**Odds Ration (OR)** is also used to compare the odds of an event occurring in one group to the odds of it occurring in another group. It is a measure of effect size.

OR = π1(1 – π1) / π2(1 – π2)

An OR of 1 means that an event is equally like in both groups two groups. An OR of < 1 means the event is less likely to occur in π2. An OR of > 1 means the event is more likely to occur in π1

## Which is appropriate for my study?

Briefly, the RR is favored in cohort studies, randomized, controlled trials, and other prospective studies. The OR is favored in case-control studies, in studies employing logistic regression, and in retrospective studies.

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