Relative risk

Relative risk (RR), or risk ratio, is an estimate of the magnitude of an association between an exposure and a disease, giving the likelihood of developing the disease in the exposed group compared with the unexposed. This is calculated as the ratio of the cumulative incidence of the disease in each group. In clinical trials, this corresponds to the probability of developing the outcome of interest in the treatment group compared with (divided by) the equivalent probability in the comparison group. A relative risk of 1 means that there is no difference in risk between the groups compared, and a relative risk of 1) means that the risk is lower (higher) in the treatment group. For relatively rare events, relative risks are similar in magnitude to odds ratios, which have statistical properties that favour their use in meta-analyses. However, risk models (that may form the basis of more complex economic models) are often built on sets of relative risks associated with different risk factors for outcomes of interest.
Also, see relative risk reducation.