When Salk measured the results, he divided the babies into three groups based on whether they had low (2510 to 3000 g), medium (3010 to 3500 g), or high (3510 g and over) birth weights. He then compared the infants from the heartbeat and silent nurseries separately within each birth-weight group. Why do you think he did that? (Remember that it would be easier to detect a difference in male and female pulse rates if all males measured 65 beats per minute and all females measured 75 than it would be if both groups were quite diverse.)