For a case-control study, participants are selected on a basis of [i]whether or not they have the disease[/i]; then, they are compared for common exposures to determine if an association exists. For a cohort study, participants are selected on a basis of [i]exposure or no exposure[/i]; then, disease occurrence is compared to determine if an association exists. Choosing a study type depends on what you already know. If there are 3 students who at school lunch last week and 100 cases of salmonella are reported, a case-control would be best because for those 100 cases, you can determine exposure to school lunch (Y/N) and look for an association.
Lyme disease, salmonellosis
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