A pharmaceutical company develops a new antihypertensive drug. Samples of 24 hypertensive patients, randomly selected from a large population of hypertensive people, are randomly divided into 2 groups of 12. One group is given the new drug over a period of 1 month; the other group is given a placebo according to the same schedule. Neither the patients nor the treating physicians are aware of which patients are in which group. At the end of the month, measurements are made of the patient’s blood pressures. This study