The excess relative risk was advocated for use as an alternative effect measure in case-control studies of rare diseases by researchers at National Taiwan University (NTU). The study was published online in PLoS ONE 2015;10(4):e0121141.
Epidemiologists often use ratio-type indices (rate ratio, risk ratio and odds ratio) to quantify the association between exposure and disease. The excess relative risk is an index involving both difference and ratio operations, but it has received little attention outside in the field of radiation epidemiology. Using the theory of sufficient component cause model, Professor Wen-Chung Lee of Institute of Epidemiology and Preventive Medicine in the College of Public Health shows that when there is no mechanistic interaction (no synergism in the sufficient cause sense) between the exposure under study and the stratifying variable for a rare disease, the excess relative risk (but not the ratio-type indices) should remain constant across strata and can therefore be regarded as a common effect parameter.
Dr. Wen-Chung Lee also pointed out two other desirable properties of the excess relative risk index, namely, it enjoys the logical properties that were previously thought to be exclusive to the ratio-type indices, and it (but not the difference-type indices, such as the risk difference) is estimable in case-control studies. Taking all these into account, researchers across all fields should seriously consider to use excess relative risk as an effect measure in case-control studies of rare diseases.