The "over-exposed" control group.

Relative risk is determined as much by the level of exposure among controls as among cases. If cases and controls are drawn from a population in which the range of exposures is narrow, then a study may yield little information about potential health effects. This may be one reason why an association between dietary fat and cancer has not been consistently observed in Western populations. Since the fat intake as a percent of total calories in the US general population varies little, only very large relative risks can be detected in epidemiologic studies. Investigators of the dietary fat hypothesis need to select study groups from populations where the risk factor is not, essentially, narrowly distributed.