How would you define controls for this study? What methods might be used to identify controls?
To test the hypothesis on the source of the outbreak, a case-control study was conducted from July 21-27. Thirty-one of the initial 38 persons meeting the original case definition (i.e., those not used in hypothesis generating interviews) were included as cases. It was decided that two controls would be selected for every case and would be matched to the case […]