As a state epidemiologist, you are called upon to determine whether a recent oil spill in a water reservoir has caused an unusual type of acute arthritis. Within the county that you work, water is supplied by two independent reservoirs separated by 100 miles. The spill occurred in one of these reservoirs. The records from the Water Department clearly indicate which households receive water from which reservoir. Finally, you develop a case definition and institute a surveillance program to detect cases. To determine an association between the oil spill and arthritis, would you perform a cohort study or case-control study