To check pain-relieving medications for potential side effects on blood pressure, it is decided to give equal doses of each of four medications to test subjects. To control for the potential effect of weight, subjects are classified by weight groups. Subjects are approximately the same age and are in general good health. Two subjects in each category are chosen at random from a large group of male prison volunteers. Subjects' blood pressures 15 minutes after the dose are shown below. Research question: Is mean blood pressure affected by body weight, by medication type, or by both?