Ogilvie et al. BMC Public Health 2010, 10:703 http://www.biomedcentral.com/1471-2458/10/703 Page 6 of 13 Figure 3 Bus on guided busway. Credit: Cambridgeshire County Council. Reproduced with permission. assumptions implicit in defining the boundaries of the ‘intervention’ and ‘control’ areas will subsequently be tested in two ways. First, the network distance from home to the nearest guided busway stop or access point will be entered as a covariate in analysis of the determinants of changes in travel behaviour in the cohort. Second, the results of two alternative approaches to analysis will be compared: one in which exposure to the intervention is defined a priori in terms of place of residence (as above), the other in which exposure to the intervention is defined post hoc in terms of actual reported use of the guided bus- way [30]. This comparison will be somewhat analogous to that between an intention-to-treat analysis and a per-pro- tocol analysis in a clinical trial. Both approaches may be