Introduction

Measuring variation in implementation of public health interventions and levels of population exposure is an important part of evaluation.1–4 In the context of effectiveness trials, exposure data can be used to interpret the impact of the intervention on the study outcomes.4 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.5

A substantial body of work exists on measuring exposure to media and other blanket health promotion campaigns.4 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, community-delivered 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.6–8 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 colleagues9 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, …