The aim of this research study was to investigate whether the Brief Addiction Therapist Scale (BATS) (Crosby, 2018), a transtheoretical fidelity measure, is associated with therapeutic outcome. The BATS evaluates therapists’ routine delivery of psychotherapies widely used in the treatment of alcohol and drug use problems. The current research conducted a secondary analysis of existing randomised controlled trial (RCT) data. Digital recordings of Motivational Enhancement Therapy (MET) sessions that were recorded as part of the original United Kingdom Alcohol Treatment Trial (UKATT Team, 2005) were observed and therapists were rated using the BATS. Multilevel statistical procedures in which the therapists were treated as a random factor were conducted in order to examine whether the BATS scores could predict therapy outcome. The results showed that the therapist BATS scores did not predict clients’ outcome of therapy. However, the analysis suggested that the BATS scores explain some of the therapist variance in outcomes. The findings of this thesis provide further support for the real-world application of the BATS.