A therapist is studying the effects of meditation and yoga on reducing stress. He offers the treatments to a random sample of 30 of his patients and lets them choose which treatment they want to use. Which of the following statements is not true about this experiment?

A random sample helps the therapist generalize results to his patient population, but it does not show which treatment caused a difference. Because patients choose their treatment, the groups may differ in ways that affect stress, and preference itself may influence outcomes. Randomly assigning patients to yoga or meditation would help balance those differences. Therefore, the statement claiming the random sample lets him determine which treatment is more effective is not true.

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