Randomised trials of complex interventions are typically designed, conducted, and analysed as if they are drug trials. Although there are many parallels there are also a number of important distinctions, which are seldom considered when designing individual trials. One of these concerns the involvement of therapists in delivering psychotherapy. Systematic reviews and meta-analyses provide an opportunity for exploring the full range and complexity of issues encountered in realistically complex situations. The first objective of the thesis was therefore to develop a conceptual framework for understanding the role of the therapist in trial designs. It was addressed by a review of the psychotherapy and statistical literatures structured according to the broad concepts of precision, internal and external validity and refined on the basis of a systematic methodological review of Cochrane reviews meta-analysing trials involving psychotherapy. The second objective was then to review, adapt, illustrate and compare methods for meta-analysing psychotherapy trials with nested designs. Methods for meta-analysing ICC estimates, absolute and standardised mean differences were adapted to allow for heteroscedasticity between treatments at the therapist- and patient- levels. These were illustrated using the example of counselling in primary care, with comparisons being made between aggregate and one-step approaches to the meta-analysis of individual-patient-data.It was argued that the therapist has two roles in randomised trials. Firstly, they are one component of a multi-component intervention, and are thus a potential treatment variable. Second, the nesting of patients within therapists creates an additional level in the design, so the therapist is also an experimental unit. The inability to conceal or randomise allocations leads to observational components within the trial design and to heteroscedasticity which deserves more attention. Characterising complex interventions, like psychotherapy, with more than one treatment variable could facilitate greater understanding of their components, how they interact, which are important, to what extent, and for whom. It also brings what is currently referred to as process research into the remit of trials, enabling a more complete evaluation of the causal effects. The broad concept of multiple experimental units makes cluster-randomised, longitudinal, multi-centre, crossover, therapist- and group-based intervention trials special cases of a more general class of multilevel trial. All involve clustering effects; their nature and the appropriate statistical model varying according to the design. Methods were proposed for the meta-analysis of continuous outcome data for two-level nested designs. A general approach was adopted, where possible, to incorporate methods covering cluster-randomised trials and the Behrens-Fisher problem. It was clear that this is a relatively untouched methodological area in need of further exploration. For the same reasons as it became necessary to summarise clinical research, it is recommended that systematic methodological reviews be carried out on a larger scale in future.

Racial and ethnic minority populations suffer disproportionally from mental health disparities in the United States (Dillon et al., 2015; Holden et al., 2014). Research has indicated that a lack of culturally competent care contributes to these disparities (Holden & Xanthos, 2009). As multicultural competencies (American Psychological Association [APA], 2002; Council of National Psychological Associations for the Advancement of Ethnic Minority Interests, 2003) have been widely endorsed and implemented in professional organizations and training programs, research on their need and usefulness has increased over the last three decades (Worthington et al., 2007). However, the majority of research on multicultural competencies has relied on analogue studies, college students, and trainees as participants (Ridley & Shaw-Ridley, 2011; Worthington et al., 2007). The current study contributed to the multicultural competency literature by including perspectives from real clients with diverse backgrounds in community settings, along with assessing therapist multicultural competency (MCC). The study examined the relationship between therapist (n = 28) multicultural competency (MCC) and psychotherapy outcomes of clients (n = 2024) from diverse racial/ethnic backgrounds in a community mental health agency. Therapist MCC did not have a statistically significant positive relationship with treatment outcome. Therapist MCC also did not have a statistically significant positive relationship with reliable or clinically significant change in treatment outcome. Results do not indicate any mediating effect of therapist MCC between race and treatment outcomes. Gender predicted overall treatment outcomes, clinically significant change and reliable change in treatment. Findings suggest cultural variables may have played a role in treatment outcomes given the differences in treatment outcomes for female clients, despite the lack of association found between therapist MCC and treatment outcomes. Implications for clinical practice and research are discussed.

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Despite the strong efficacy of couple therapy, many couples still do not benefit from treatment. Marriage and family therapy scholars have argued that therapists play a crucial role in the delivery of successful couple therapy, yet little research has documented that the therapist in couple therapy has a significant impact on outcomes. Known as the study of therapist effects, this study sought to assess the amount of variance attributed to the therapist in couple therapy outcomes. Using dropout as the outcome variable, this study analyzed data from 1192 couples treated by 90 therapists at a university-based training clinic. Results from multilevel analysis indicated that therapists in the sample accounted for 9.5% of the variance in couple dropout while controlling for initial couple impairment. Therapist gender and therapist experience did not significantly predict the effectiveness of therapists. These findings give promise to future research on therapist effects in couple therapy and encourage exploration into which therapist characteristics and behaviors contribute to successful clinical outcomes.

Functional analyses (FAs) are a common tool used in the assessment and treatment of severe problem behaviors and often occur in the context of clinical settings with unfamiliar, trained staff. Previous research suggests that inconsistent outcomes can emerge when caregivers with an existing history of seeing their child’s challenging behavior are trained to implement the assessment in place of clinical staff. The purpose of the current study was to expand on existing literature by comparing FA implemented by clinical staff and caregivers in the context of a clinical setting. Results demonstrate that efficient identification of function and differentiated rates of problem behavior given the inclusion of caregivers during assessment may vary based on the child’s existing history of responding with those caregivers. Implications of results for researchers and practitioners are discussed.