A physician is testing the effectiveness of two new exercise plans on 70 subjects. To assign the subjects to the two treatment groups, he writes "Plan 1” on one slip of paper and "Plan 2” on another slip of paper. He randomly picks one of the slips, and assigns the first 35 subjects who signed up to that plan, and assigns the last 35 subjects to the other plan. Select the statement that best describes the composition of the treatment groups.
Answer
A
The treatment groups might not be comparable, because there is not a control group.
B
The treatment groups should be comparable, because there are a large number of subjects in each group.
C
The treatment groups should be comparable, because the exercise plan for each group was randomly selected.
D
The treatment groups might not be comparable, because the first 35 subjects to sign up for the study might share a characteristic related to exercise, such as high motivation.
Explanation
Although a slip randomly determines which plan the first 35 subjects receive, the subjects themselves are not randomly assigned to groups. The first 35 people to sign up may differ from the last 35 in a way related to exercise, such as motivation, so the groups might not be comparable. Therefore, the best choice is that the first 35 subjects might share a relevant characteristic.
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