Biological interpretation of relative risk.

There is widespread interest in assessing the clinical importance of a study result. This goal is impeded, however, by a lack of clarity about the biological interpretability of epidemiological effect measures, such as the relative risk. A relative risk is often interpreted merely as a measure of some vague statistical association, without a view toward a biological effect as an object of measurement. Not infrequently, if it is not statistically significant, the relative risk estimate is ignored completely. A key to biological interpretation is appreciating the theoretical framework stipulating that outcome rates derived from 2 comparison groups actually represent measures of different effects in the same population. For instance, by using a placebo group to estimate the number of background cases that occurred in the treatment group, an estimate of the number of excess cases that occurred as a result of treatment can be made. This kind of biological entity can be derived from a relative risk, and can be more easily evaluated as to its clinical importance than a statistical association or a statement about statistical significance. Interpretation then becomes a more directed task, with a focus on the validity of certain ancillary hypotheses upon which biological interpretability rests.