What factors make it challenging to accurately assess the effectiveness of psychotherapeutic interventions?

Explanation

Evaluating psychotherapy is complicated because patients often self-select into therapy, causing biased samples (A). Additionally, forming authentic control groups is challenging (B), and the numerous combinations of treatments and disorders make comprehensive assessment difficult (C). While patient consent can be an issue, it is less significant compared to these factors (D). Therefore, options A, B, and C collectively explain the difficulties.