Miettinen and Caro (J Clin Epidemiol 1989; 42: 325-331) (6] put forth principles of non-experimental assessment of excess risks in case-base studies; in such a design, risk difference is assessed by estimating the denominators of the proportions of cases among exposed and comparable unexposed subjects by means of a representative sample from the base population of the study.

They provided appropriate formulations for the point estimation of risk differences for various exposure patterns with allowance for covariables by means of stratified analysis.