A pharmacy resident completes a retrospective study of patients in a clinic who were prescribed amiodarone. The resident examines the medical profile of each patient to determine if the amiodarone caused hyperthyroidism. He then compares the risk of hyperthyroidism to a group of patients who did not receive amiodarone. This is an example of what type of study? 1) Cohort 2) Randomized controlled trial 3) Cross-sectional 4) Case-control 5) Crossover