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