The effectiveness of biofeedback has been a subject of controversy because biofeedback __________.

Biofeedback is difficult to evaluate scientifically because it can be hard to design an effective test that separates the effect of the feedback from other factors, such as practice or expectations. Therefore, the best choice is “cannot be effectively tested.” The absence of reported side effects or negative effects does not explain the controversy, and uncomfortable electrodes are not the central issue.

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