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A pharmaceutical company tests a new medication on two groups: 200 people who already use a competitor's medication, and 200 people who use no medication. After 8 weeks, the new medication shows greater improvement in the new-user group. What is the primary concern with this experimental design?
A pharmaceutical company tests a new medication on two groups: 200 people who already use a competitor's medication, and 200 people who use no medication. After 8 weeks, the new medication shows greater improvement in the new-user group. What is the primary concern with this experimental design?