Measures of exposure to the Well London Phase-1 intervention and their association with health well-being and social outcomes

To cite: Phillips G, Bottomley C, Schmidt E, et al. J Epidemiol Community Health 2014;68:597–605. INTRODUCTION Measuring variation in implementation of public health interventions and levels of population exposure is an important part of evaluation. In the context of effectiveness trials, exposure data can be used to interpret the impact of the intervention on the study outcomes. In interventions where participation is voluntary or self-selected, measures of exposure can highlight inequity of access to the intervention among population subgroups of interest in public health. A substantial body of work exists on measuring exposure to media and other blanket health promotion campaigns. Less attention has been paid to measuring ‘exposure to’, or perhaps more appropriately ‘participation in’, complex public health interventions delivered through community engagement approaches. During the 1980s and 1990s, a handful of large-scale, multicomponent, communitydelivered public health interventions were conducted in the USA, and evaluations of these programmes included extensive process measures to estimate the levels of exposure to and/or participation in different components of the intervention, ranging from mass media to small-group health education classes. These studies used the amount of potential exposure events or participation time offered and the actual number of exposure events or duration of participation as measures of implementation. They also attempted to weigh exposure to different intervention elements according to theoretically derived estimates of their relative effectiveness. More recently, Oakley and colleagues have suggested that quantitative and qualitative process evaluation of exposure to interventions should be considered an essential component of randomised controlled trials of complex public health interventions. This is because they can help, inter alia, to monitor the dose and reach of the intervention, to investigate contextual factors that influence effect, to explore the relationship between trial outcomes and variation in the extent and quality of implementation and to identify processes that might mediate the relationship between intervention and outcomes. However, high-quality process evaluation and measurement of exposure to intervention in complex public health interventions that use community engagement approaches is often difficult to achieve and difficult to resource. In this paper, we describe the measures of intervention exposure used in the cluster randomised trial of the Well London programme, a public health intervention using community engagement and community-based projects to increase physical activity, healthy eating and mental health and wellbeing in 20 of the most deprived neighbourhoods in London. No earmarked resources to support the development of these measures and associated data collection were provided to either the research team or to those delivering the interventions on the ground. Instead, these were derived from contractually specified performance management information reported quarterly by partners and by inclusion of questions seeking information about participation in the follow-up questionnaires used to measure the main trial outcomes. The exposure measures are consequently considerably less sophisticated than those used in the US studies, where earmarked funding was available. The main trial analysis did not provide evidence of neighbourhood-level effects on primary outcomes arising from delivery of the Well London programme, while the nested qualitative study indicated that active participation in the programme was key to improvements in subjective well-being. This paper aims (a) to describe the measures of exposure to Well London available for the trial analysis, (b) report the levels (and where possible sociodemographic correlates of) exposure and (c) examine the relationship between the trial outcomes and exposure measures. These analyses were prespecified in our trial analysis plan. In addition, we consider the limitations of our exposure measures and the importance and methodological challenges of measuring exposure in the evaluation of a community-level intervention.