Investigator at beginning of study

    ?        To be determined

At the beginning of the case control study, the investigator knows that there are some people with a disease; they are matched with similar individuals (controls) who do not have the disease. The investigator looks backward to identify what different exposures the two groups might have had. For example, when some individuals attending a picnic become ill, they could be matched with controls who also attended the picnic but did not become ill, and all interviewed about what was eaten, to identify a possible source of food-borne illness.