Retrospective studies have always taken for granted that matching should be done on factors which affect the incidence of the disease. Worcester stated that when that "when a disease group is being compared with another group matching is usually done on variables known to be related to the disease rather than on variables related to the outcome." Miettinen et al. however disagrees. They believe that factors on which matching should be done must be related to the outcome variable otherwise they do not affect the measure that is the basis for a decision about the association between the putative etiologic agent and the disease. Thus in a retrospective study of the association between blood group and cervical cancer male controls would be quite acceptable if they were healthy and were matched to the patients on race. The erroneous view that one should match on factors which affect the incidence of disease may have resulted from the confusion with follow-up studies where it is proper to match on factors that are correlated with the disease since the occurrence of disease is the outcome variable in follow-up studies. Miettinen further states that one should match only on those factors that are correlated with both the outcome variable and disease incidence. The authors state that whenever a factor is strongly correlated with the outcome one should match on this factor or consider it in the analysis even if a priori it is thought not to affect the incidence of the disease. In cases where there is uncertainty whether a variable is or is not correlated with the outcome variable the decision to match or not may be influenced by whether the variable is kown to affect the disease incidence.