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Old 04-06-2013
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Stats Case Control vs Cohort Study?

From October 1997 through October 1998, 16 outbreaks of acute gastrointestinal illness (illness involving portions of the gastrointestinal tract, including the stomach, and small and large intestines) occurred in seven states, Florida, Georgia, Illinois, Indiana, Kansas, North Dakota, and Pennsylvania. All but one outbreak occurred in schools, and most of the approximately 1,700 people affected were children. The Disease Detectives investigating this problem included epidemiologists, physicians, food safety experts, laboratory specialists, and other scientists.

On Monday, March 23, 1998, teachers at Putz Elementary School in Hall County, Georgia, noted that many pupils became ill with nausea and vomiting during and shortly after lunch. The school administrators suspected food poisoning, and at about 1:30, the school dietician, Mr. Wholesome, called the Hall County health department and spoke with the local Disease Detective, Mr. Sleuth, who is an environmental health specialist. Mr. Sleuth arrived at the school at about 2:00 p.m. and immediately began an investigation. Putz Elementary School is in a rural area, serves primarily a lower socioeconomic population, and has approximately 600 pupils in 28 classes, kindergarten through fifth grade.

Mr Sleuth interviewed the school principal and four students and discovered that for these students, illness occurred less than an hour after eating. Based on this preliminary information, Mr. Sleuth developed a questionnaire and on March 24 administered this to all fifth graders. The questionnaire asked for identifying data, symptoms, time of onset of illness, and foods eaten for lunch on March 23. Further data collection included interviews of all students to determine illness.

On March 23, Mr. Sleuth collected vomitus samples from five students and samples of food from the trash for laboratory analysis. He interviewed kitchen staff members about food handling procedures and the sources of food served that day. The kitchen was inspected as well. Mr. Smith sent the food and vomitus samples to Ms. Beaker at the Georgia Public Health Laboratory. Ms. Beaker tested these materials for several different organisms that commonly cause food poisoning.

Of all students interviewed, 452 provided information considered reliable about both illness and foods eaten. Of these, 155 (34%) had become ill following the lunch on March 23. Symptoms reported by fifth graders were nausea (89%), headache (65%), abdominal pain (53%), vomiting (29%), and diarrhea (17%). The duration of illness for fifth graders ranged from 10 minutes to 8 hours with a median of 4.5 hours.

The children had a choice of eight food and beverage items at lunch. Information on fifth graders who ate the food and beverage items and on those who did not is summarized in the table below.

Table. Foods eaten and beverage consumed at lunch by fifth grade classes Putz Elementary School, Gainesville, Georgia, March 23, 1998

The table is attached. What I'm confused with is why this study is Cohort, when there are defined cases and controls and the presence of a disease is established and then the exposures are checked.
Thanks for your help!
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Old 04-06-2013
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This clearly is a Cohort study

Where is the control group here ?
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