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 National Institutes of Health gave the drug to 150 sickle-cell sufferers and a placebo to another 150 sufferers. The researchers then counted the number of episodes of pain reported by each subject. What is the response variable in this study