Researchers conduct a study to determine whether a drug used to treat hypertension in patients with obstructive sleep apnea works better when taken in the morning or in the evening. To perform the study, 78 patients are given one pill to take in the morning and one pill to take in the evening (one containing the drug and the other a placebo). After 6 weeks, researchers collected blood pressure information on the patients.

Determine whether the study is an observational study or an experiment. Explain.