In addition to the measures of association based on differences between means, proportions, or rates, Epidemiology contributes 2 measures of association based on ratios.

Relative Risk (RR) is the cumulative incidence in the exposed population divided by the cumulative incidence in the unexposed. It is a direct comparison of probabilities. Preferred measure.

Relative Prevalence (RP) - When the data are based on prevalence rather than incidence. The comparison is between two probabilities of “having” a health status outcome.

Odds Ratio (OR) - the odds of having the outcome in the exposed divided by the odds of having the outcome in the unexposed. It approximates the relative risk when the health outcomes are rare. Not a direct comparison.