Risk Ratio (RR)
The Relative Risk(RR) of an event, such as the occurrence of a specified disease or a death from a specified cause, is the ratio of the risk of a disease or death among those exposed to a specified factor to the risk among those not exposed to this factor. It is calculated from the incidence of the specified disease or the death rate due to the disease.
If the level of risk in both the exposed and unexposed group is the same, the RR will equal 1. If an exposure is harmful (e. g., cigarette smoking), the RR is expected to be greater than 1. If an exposure is protective (e. g., vaccine), the RR will be less than 1. The RR is used in randomized controlled trials and cohort studies .
The RR is less relevant to making decisions in risk management than attributable risk. Nevertheless, relative risk is the measure of association most often used by epidemiologists.