Relative risk, usually abbreviated RR, is a comparison of risk levels between two groups in a study, usually the treatment and the control group. Relative risks are similar to, but not exactly the same as hazard ratios, though they are often reported the same way. A relative risk of 1 indicates no change in risk between the exposed group and the control group. Relative risks over 1 indicate that a treatment or exposure increases risk. Relative risks under one indicate decreased risks. For example, a relative risk of 2 indicates a doubling of average risk. While a relative risk of .75 means that that the average risk dropped by 25 percent.
