A study found that patients referred by their doctors to psychotherapists practicing a new experimental form of therapy made more progress with respect to their problems than those referred to psychotherapists practicing traditional forms of therapy
Therapists practicing the new form of therapy, ttherefore, are more effective than therapists practicing traditional forms
It ignores the possibility that therapists practicing the new form of therapy systematically differ from therapists practicing traditional forms of therapy with regard to some personality attribute relevant to effective treatment.