Psychotherapy's equivalence paradox is that treatments tend to have equivalently positive outcomes despite non-equivalent theories and techniques. We replicated an earlier comparison of treatment approaches in a sample four times larger and restricted to primary-care mental health.

Patients (n=5613) who received cognitive–behavioural therapy (CBT), person-centred therapy (PCT) or psychodynamic therapy (PDT) at one of 32 NHS primary-care services during a 3-year period (2002–2005) completed the Clinical Outcomes in Routine Evaluation – Outcome Measure (CORE-OM) at the beginning and end of treatment. Therapists indicated which approaches were used on an End of Therapy form. We compared outcomes of groups treated with CBT (n=1045), PCT (n=1709), or PDT (n=261) only or with one of these plus one additional approach (e.g. integrative, supportive, art), designated CBT+1 (n=1035), PCT+1 (n=1033), or PDT+1 (n=530), respectively.

All six groups began treatment with equivalent CORE-OM scores, and all averaged marked improvement (overall pre/post effect size=1.39). Neither treatment approach nor degree of purity (‘only’ v. ‘+1’) had a statistically significant effect. Distributions of change scores were all similar.

Replicating the earlier results, the theoretically different approaches tended to have equivalent outcomes. Caution is warranted because of limited treatment specification, non-random assignment, incomplete data, and other issues. Insofar as these routine treatments appear effective for patients who complete them, those who fail to complete (or to begin) treatment deserve attention by researchers and policymakers.