Therapist effects and moderators of effectiveness and efficiency in psychological wellbeing practitioners: A multilevel modelling analysis.

Objectives: The study investigated whether psychological wellbeing practitioners (PWPs) working within the UK government's Improving Access to Psychological Therapies (IAPT) initiative are differentially effective (i.e., therapist effect size) and differentially efficient (i.e., rate of clinical change), and the moderating effect of demographic and process factors on outcomes.

Design And Methods: Routine clinical outcome data (depression, anxiety, and functional impairment) were collected from a single IAPT service. A total of 6111 patients were treated by 56 PWPs. Multilevel modelling (MLM) determined the size of the therapist effect and examined significant moderators of clinical outcomes. PWPs were grouped according to below average, average, and above average patient outcomes and compared on clinical efficiency.

Results: Therapist effects accounted for 6-7% of outcome variance that was moderated by greater initial symptom severity, treatment duration, and non-completion of treatment. Clinically effective PWPs achieved almost double the change per treatment session. As treatment durations increased beyond protocol guidance, outcomes atrophied. Treatment non-completion was particularly detrimental to outcome.

Conclusions: PWPs appear to be differentially effective and efficient despite ostensibly delivering protocol driven interventions. Implications for services, training, and supervision are outlined.