Why does treatment fidelity matter?

Treatment fidelity is a relatively new concept in intervention research and has been discussed in scientific literature only in the last 25 y. Intervention fidelity improves power by reducing unintended variability in treatment effect, and it supports external validity by allowing for replication. As more self-administered complementary and alternative medicine (CAM) treatments are being studied, the issue of fidelity concerns is becoming more of a burden to researchers. Despite the critical role of fidelity, no comprehensive, structured guide exists.