you carry out a clinical trial to test whether a new drug relieves the symptoms of arthritis better than a placebo does. you have four groups of participants, all of whom have mildly painful arthritis (rated 6 on a scale of 1 to 10). each group receives a daily pill as follows: group 1 (control), placebo; group 2, 15 mg; group 3, 25 mg; group 4, 50 mg. at the end of 2 weeks, participants in each group are asked to rate their pain on a scale of 1 to 10. what is the independent variable in this experiment?