Interpreting results: Relative risk and odds ratio

Relative risk and difference between proportions

The most important results from analysis of a 2x2 contingency table is the relative risk, odds ratio and difference between proportions. Prism reports these with confidence intervals.

In this example, disease progressed in 28% of the placebo-treated patients and in 16% of the AZT-treated subjects.

The difference between proportions (P1-P2) is 28% - 16% = 12%.

The relative risk is 16%/28% = 0.57. A subject treated with AZT has 57% the chance of disease progression as a subject treated with placebo. The word “risk” is not always appropriate. Think of the relative risk as being simply the ratio of proportions.

Odds ratio

If your data represent results of a case-control retrospective study, choose to report the results as an odds ratio. If the disease or condition you are studying is rare, you can interpret the Odds ratio as an approximation of the relative risk. With case-control data, direct calculations of the relative risk or the difference between proportions should not be performed, as the results are not meaningful.