During an outbreak investigation the team wants to formally test whether attendance at a campus event is associated with sickness, without assigning any intervention. Which study type should they choose?

• A. Ecological study that aggregates exposure and outcome data at a population level without individual-level comparisons
• B. Experimental study in which investigators randomly assign people to attend or not attend the event and then compare outcomes
• C. Analytical (observational) study such as a case-control or cohort study
• D. Descriptive study where investigators only summarize counts and rates by age, time, and location and do not compare exposure distributions between groups

Answer: (C)