Sickle-cell disease is a painful disorder of the red blood cells that in the united states affects mostly african americans. to investigate whether the drug hydroxyurea can reduce the pain associated with sickle-cell disease, a study by the nih gave the drug to 150 sickle-cell sufferers and placebo to another 150. neither doctors nor patients were told who received the drug. the number of episodes of pain reported by each subject was recorded. this is an example of a(n):