Odds Ratio

Definition

An odds ratio (OR) is a measure of association between an exposure and an outcome (i.e., disease) and is commonly derived from data collected in a case–control study design. When the outcome is rare, the OR is a good estimate of the relative risk of the outcome. The absolute value of the OR varies in size from 0 to infinity; its size is an indication of the magnitude of the association. An OR >1.0 indicates that the likelihood of the exposure is increased in the presence of the outcome. For example, an OR of 3.00 is interpreted as meaning that those with the outcome are three times as likely to be exposed compared to those without the outcome. An OR <1.0 indicates that the likelihood of the exposure is decreased in the presence of the outcome, which is often referred to as a “protective effect.” For example, an OR of 0.70 is interpreted as meaning that persons with the outcome are 1.00–0.70, or 30%, less likely to be exposed compared to persons without the outcome. Since a...