Study Design Classification
This is a case-control study (Option A). The researcher is comparing individuals who became ill (cases) with those who did not become ill (controls) to retrospectively identify which food exposures were associated with the outbreak 1, 2.
Defining Characteristics of This Investigation
The study design is definitively case-control because:
Cases are selected based on outcome status (those who developed food poisoning) and compared to controls (those who did not become ill), which is the fundamental structure of case-control methodology 1, 3, 4.
The investigation looks backward in time from the outcome (illness) to identify exposures (specific foods consumed), which is the hallmark "looking back" approach of case-control studies 3, 4, 5.
Participants are recruited after the outbreak has already occurred, with cases identified through their illness status rather than being followed forward from the time of exposure 1, 2.
Why This Is NOT a Cohort Study
Cohort studies follow populations forward in time from exposure to outcome, tracking who develops disease among exposed versus unexposed groups 6. In this scenario, the researcher did not identify festival attendees before they ate and then follow them to see who became ill—instead, they identified ill people first and worked backward 6.
The temporal sequence is reversed: In a cohort study, you would identify all festival attendees at the time of the event, document what each person ate, and then follow them prospectively to determine who develops illness 6. Here, the illness has already occurred and the investigation is retrospective 1, 2.
Why This Is NOT a Randomized Controlled Trial
RCTs involve deliberate intervention and random allocation of exposures by researchers 6. Obviously, investigators cannot and do not randomly assign people to eat potentially contaminated food during an outbreak investigation 6.
Practical Application in Outbreak Investigation
The CDC recommends this case-control approach specifically for foodborne outbreak investigations to rapidly identify the implicated food vehicle 2. The methodology involves:
Defining cases using clinical criteria (e.g., diarrhea, vomiting, specific symptom onset timing) 1, 2.
Identifying controls from the same population who attended the festival but remained well 1, 2.
Conducting structured interviews about all specific food items consumed 2.
Calculating odds ratios for each food exposure to quantify associations—as demonstrated in real outbreak investigations where odds ratios as high as 65.0 to 84.0 have identified specific contaminated foods 1, 2.
Advantages for Outbreak Investigation
Case-control studies are particularly appropriate for investigating disease outbreaks because they are relatively quick to perform, economical, and can be implemented rapidly when immediate public health action is needed 5. This design allows investigators to efficiently identify risk factors without the time and resource requirements of prospective follow-up 4, 5.