Question

Control subjects for the case-control study were randomly selected from local telephone directories and were matched to case-patients according to the following criteria: the same sex and telephone exchange, and age within 10 years of the corresponding patient if the patient was 30 years of age or older, and within 5 years if the patient was under 30 years of age. Just as the case-control study was getting under way, a new case-patient was diagnosed in Sioux Falls and was interviewed by some members of the investigative team. The woman, age 25, clearly had painless thyroiditis and hyperthyroidism. She lived in Sioux Falls but frequently visited a grocery store in Valley Springs, S.D., which her father owned. She sometimes purchased groceries there. Team members went to the store and interviewed the proprietor. All goods in the store were obtained from national distribution systems except two –chicken eggs and beef trimmings. The beef trimmings were obtained from a plant near Luverne. Some team members began to suspect that ingestion of beef may have been the source of the outbreak. They hypothesized that iodine contaminated the beef trimmings during processing, where it may have been used as a disinfectant. Then, during the case-control study, one investigator visited a family of four people where all members of the family except one had illness fulfilling the case definition. The one unaffected member was a young boy who was a vegetarian. The family obtained its meat from the Luverne beef plant. The results of the case-control study became available. The study showed that two factors were associated with illness – consumption of commercially processed chicken (odds ratio 2.3, p=0.03) and consumption of ground beef prepared by the Luverne plant (odds ratio 1.9, p=0.05). However, during the study, case- patients were not asked about the source of their beef in a uniform way; some team members suspected information bias

Question: How would you obtain further data to test the hypothesis that ingestion of beef or poultry is the cause of the outbreak?

Answer 1

Page order 3,1,4,2
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