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.

Children’s risk for overweight and obesity is particularly high in rural areas of the United States. Many health, psychosocial, and economic consequences are associated with childhood overweight and obesity, which concerns health researchers and professionals. But how and why might rural children be more at risk for being overweight and obese? This dissertation investigates childhood overweight and obesity in rural settings through three separate studies. First, a systematic literature review was conducted to identify determinants and mechanisms of childhood obesity-related behaviors that are specific to rural locations. The findings from the review show that lack of health resources and poverty within the rural environment may impact children’s social environment and individual factors. However, results are inconclusive and there continues to be a lack of studies focusing on linking environmental influence with individual factors. Second, a meta-analysis of current research evidence was conducted to assess the efficacy of rural interventions designed to reduce childhood overweight and obesity. Results showed that interventions have been efficacious yet modest, with a mean effect size of 0.18. Moderating variables were also examined. Mean intervention effect size was moderated by children’s age and intervention duration. Last, secondary data were used to examine the association between rural food stores and availability and affordability of fresh fruits and vegetables. A multilevel analytical approach was used to determine if rural location was associated with availability and affordability of fresh fruits and vegetables. After controlling for other variables, results showed that rural location was not associated with fruit and vegetable availability and affordability. The findings from this dissertation suggest that the area of rural childhood overweight and obesity remains understudied. More research is needed in order to understand the mechanisms of social ecological influences on diet, physical activity, and childhood overweight and obesity. This area of research, however, is rife with opportunities for public health education and promotion. Public health educators can help promote and advocate for environmental conditions that support healthy lifestyles.