Observational studies6/1/2023 This action was in direct response to the convincing results of the analytic epidemiology, which compared the exposure history of case-patients with that of an appropriate comparison group. Shortly thereafter, the Food and Drug Administration issued an advisory to the public about green onions and risk of hepatitis A. Further investigation of the ingredients in the salsa implicated green onions as the source of infection. ![]() Of 133 items on the restaurant’s menu, the most striking difference between the case and control groups was in the proportion that ate salsa (94% of case-patients ate, compared with 39% of controls). The investigators, therefore, also enrolled and interviewed a comparison or control group - a group of persons who had eaten at the restaurant during the same period but who did not get sick. The investigators asked the case-patients which restaurant foods they had eaten, but that only indicated which foods were popular. ![]() While the investigators were able to narrow down their hypotheses to the restaurant and were able to exclude the food preparers and servers as the source, they did not know which particular food may have been contaminated. Consider a large outbreak of hepatitis A that occurred in Pennsylvania in 2003.( 38) Investigators found almost all of the case-patients had eaten at a particular restaurant during the 2–6 weeks (i.e., the typical incubation period for hepatitis A) before onset of illness. The key feature of analytic epidemiology is a comparison group.
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