The authors of studies often report their results using abbreviated terms such as RR, OR, ARR, RRR and NNT. These terms are quantities that express the strength of association between the dependent and independent variables and are collectively referred to as
measures of association. The similarity between these measures and the multiple terms by which each is referred can be confusing. The purpose of this article is to explain in a straightforward manner the purpose, derivation, and limitations of some of the more commonly used categorical measures of association, including relative risk, odds ratio, absolute and relative risk reduction and number needed to treat, using results from recent emergency medicine studies published by Canadian researchers.