investigators start byenrolling a group of people with disease (at CDC such persons arecalled case-patients rather than cases, because case refers tooccurrence of disease, not a person). As a comparison group, theinvestigator then enrolls a group of people without disease(controls). Investigators then compare previous exposures betweenthe two groups. The control group provides an estimate of thebaseline or expected amount of exposure in that population. If theamount of exposure among the case group is substantially higherthan the amount you would expect based on the control group, thenillness is said to be associated with that exposure. The study ofhepatitis A traced togreen onions, described above, is an exampleof a case-control study. The key in a case-control study is toidentify an appropriate control group, comparable to the case groupin most respects, in order to provide a reasonable estimate of thebaseline or expected exposure