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A physical therapy clinic offers a new rehabilitation program for ACL injuries. Interested patients can self-select and enroll in the program. After 6 months, researchers compare 40 patients who completed the program to 40 patients on the clinic's waiting list (control group). Results show the program group has significantly better knee function and strength scores.
Which statement most accurately describes the threat to validity in concluding the program is effective?
A physical therapy clinic offers a new rehabilitation program for ACL injuries. Interested patients can self-select and enroll in the program. After 6 months, researchers compare 40 patients who completed the program to 40 patients on the clinic's waiting list (control group). Results show the program group has significantly better knee function and strength scores.
Which statement most accurately describes the threat to validity in concluding the program is effective?