Measures of association based on relative differences in risk: relative risk and odds ratio

To assess the relationship between exposure and outcome is the main purpose of epidemiological studies. The measures of association evaluate the direction and magnitude of this relationship. Relative risk (RR) and odds ratio (OR) are the measures used most when the strength of the association between an exposure and an outcome is measured in relative terms. RR is calculated as the ratio of the incidence rate of a certain outcome among individuals exposed to a particular factor and the incidence rate among those not exposed to the same risk factor. OR is calculated as the ratio of the odds that an exposed person develops disease and the odds that an unexposed person develops disease. RR and OR higher than 1 mean that exposure is a risk factor for the outcome. RR and OR lower than 1 mean that exposure is a protective factor for the outcome. RR and OR equal to 1 mean that exposure is not associated with the outcome.