Background. Psychotherapy's equivalence paradox is that treatments have equivalently positive outcomes despite non-equivalent theories and techniques. We compared the outcomes of contrasting approaches practised in routine care.

Method. Patients (n=1309) who received cognitive-behavioural therapy (CBT), person-centred therapy (PCT) and psychodynamic therapy (PDT) at one of 58 National Health Service (NHS) primary and secondary care sites in the UK during a 3-year period completed the Clinical Outcomes in Routine Evaluation-Outcome Measure (CORE-OM) at the beginning and end of their treatment. Therapists indicated which treatment approaches were used on an End of Therapy form. We compared outcomes of six groups: three treated with CBT, PCT or PDT only, and three treated with one of these plus one additional approach (e.g. integrative, supportive, art), designated CBT+1, PCT+1 or PDT+1 respectively.

Results. All six groups averaged marked improvement (pre–post effect size=1·36). Treatment approach and degree of purity (‘only’ vs. ‘+1’) each accounted for statistically significant but comparatively tiny proportions of the variance in CORE-OM scores (respectively 1% and 0·5% as much as pre–post change). Distributions of change scores were largely overlapping.

Conclusions. Results for these three treatment approaches as practised routinely across a range of NHS settings were generally consistent with previous findings that theoretically different approaches tend to have equivalent outcomes. Caution is warranted because of limited treatment specification, non-random assignment, lack of a control group, missing data and other issues.