Research Question: What is the combined effect of medication and diet on blood pressure in adult males?

A randomized, controlled experiment investigated this question using a random sample of 540 adult males as subjects. At the beginning of the study, researchers measured the blood pressure of all the subjects. Each subject was given a month's supply of either Drug A, Drug B, or a placebo, then was assigned to be on either a low-fat diet or a low-salt diet for that month-long period. At the end of the month, the researchers measured the blood pressure of each subject again, and compared the two blood pressure readings.

1. How many different groups are there altogether in this study?
   
   
   • 2
   • 3
   • 4
   • 5
   • 6
   
   
   
2. Which of the following is/are the explanatory variable(s) in this study? Check all the boxes that apply.
   
   
   • Blood pressure before the study
   • Blood pressure after the study
   • Difference in the two measurements of blood pressure
   • Type of drug given
   • Type of diet