How does online therapy work?: Exploring skills usage as a mechanism of change in internet-delivered cognitive-behavioural therapy for depression and anxiety

Major depression and generalised anxiety disorder (GAD) are common disorders that are often comorbid. Cognitive-Behavioural Therapy (CBT) is an efficacious treatment, which can be delivered online (iCBT), but little is known about its mechanism of therapeutic change. Understanding mechanisms will help improve treatment efficacy and efficiency. A core feature of CBT is teaching patients skills that promote adaptive cognitions and behaviours. However, there is limited evidence indicating that skills usage is a mechanism of change during CBT. The present thesis aims to evaluate the mechanistic role of CBT skills usage on symptoms of depression and GAD during iCBT. The aim of study one was to develop a brief questionnaire assessing CBT skills usage suitable for mechanism research. Responses from 661 participants were subjected to exploratory and confirmatory factor analysis yielding a 12-item measure of CBT skills usage, the Frequency of Actions and Thoughts Scale (FATS). In study two, the FATS was administered during an iCBT course for depression and anxiety (n = 125). Large increases in FATS scores were observed from pre-treatment to post-treatment (d = 0.90), and residualised change scores correlated significantly with residual change scores of symptoms (rs = .28 - .30). The third study utilised a randomised controlled trial investigating the causal relationship between treatment and skills usage. Results indicated that patients receiving iCBT for depression and anxiety (n = 65) reported significantly greater FATS scores (d = 0.58) at the end of treatment compared with a wait-list control group (n = 66). Mediation analysis revealed that symptom reduction was mediated by increased skills usage at the end of treatment. Study four pooled the data from studies two and three, which was used to explore predictors of skills usage. Overall, the results from this research suggest that skills usage is an important mechanism of change in iCBT for depression and GAD.