We agree with Dr. Messori that some study designs (e.g., "case-based" case-control studies) do not permit the calculation of absolute measures of effect. However, because some types of case control studies do permit the calculation of absolute effect measures (e.g. nested case-control studies)[1,2] we did not exclude this term from our search strategy. Among our sample of 344 articles we only found 6 case-control studies, thus we feel that any bias due to their inclusion would not affect our substantive conclusions.

References
1. Langholz B, Borgan O. Estimation of absolute risk from nested case-control data. Biometrics. 1997;53(2):767–774.
2. Austin PC. Absolute risk reductions and numbers needed to treat can be obtained from adjusted survival models for time-to-event outcomes. J Clin Epidemiol. 2010;63(1):46–55

Competing interests: None declared