Ogilvie et al. BMC Public Health 2010, 10:703 http://www.biomedcentral.com/1471-2458/10/703 recorder and subsequently transcribed verbatim. Partici- pants who complete an interview at baseline may be invited to take part in a follow-up interview later in the study, along with other participants purposively selected on the basis of their responses to the core questionnaire survey. Photo-elicitation interviews A subset of the interview sample (n~20) is invited to take part in a follow-up inter- view several months later. After being given guidance on the rights and responsibilities associated with taking photographs [56], participants are asked to take photo- graphs of either their actual or their ideal commuting journey using disposable cameras [57-59] or their own digital cameras. The subsequent interview is mainly guided by participants’ discussion of these photographs which serve as self-generating prompts. Photo-elicitation is widely used as a research tool in qualitative health research [57,58,60] to add a participatory element and also to facilitate in-depth interviewing. In this study, it also enables the project to capture potential seasonal and other changes in travel behaviour. Finally, participant- produced photographs form visual data of the physical and social contexts in which travel takes place that can be analysed in their own right. Participants may choose to take photographs as a family or household project or in collaboration with work colleagues, in which case other household members or work colleagues may be present during the photo-elicitation interviews and infor- mally contribute to the interview conversation [61]. Some photo interviews are also combined with a ‘map- drawing’ exercise for further elicitation purposes and to locate photos or interview narratives along travel routes. Further ethnographic data Interviews and photo-elici- tation interviews are complemented by detailed ethno- graphic field notes written by the researcher, during and immediately after each interview. Field notes document the context in which interview data is produced, record other informal interactions and gather data on the environmental and social surroundings of participants’ homes and journeys. Ethnographic data also include general observation of travel practices in the local area and local public discourses around travel, in order to further contextualise participants’ representations of tra- vel behaviour and social and environmental factors. Analysis Quantitative outcome measures The study was originally designed on the basis of a pri- mary outcome measure defined in terms of the net change in time spent walking or cycling on the journey to and from work (daily active commuting time) after one year (min/day). In addition, the study was originally designed to examine the following secondary outcome measures: Page 9 of 13 1. Net change in active commuting time after two years (min/day) 2. Net change in total active travel time after one and two years (min/day) 3. Net change in overall physical activity expenditure estimated from self-reported data after one and two years (metabolic equivalent (MET)-h/week based on RPAQ data) 4. Net change in overall physical activity estimated from objective measurement data after one and two years (mean counts/min, and min/week spent in MVPA) 5. Net change in wellbeing after one and two years (changes in SF-8 physical and mental health summary scores) 6. Net change in self-reported sickness absence after one and two years (days/year). Net change is defined as the average within-subject change in a given outcome measure in the ‘intervention’ group minus the average within-subject change in the ‘control’ group after adjustment for demographic, socioeconomic, geographical, psychosocial and health correlates of travel behaviour at baseline using multivariate regression analysis. Change in these outcome measures will be studied over a two-year follow-up period in the majority of the cohort, but owing to the delayed implementation of the interven- tion it is likely that the greatest impact (if any) of the bus- way on these outcome measures will occur between the second and third years of data collection, and not between the first and second years as originally intended. Sample size estimation Data obtained in the validation study for RPAQ [40] suggest mean daily active commuting times in Cam- bridge of about eight minutes per day for men and five for women; various studies suggest that the standard deviation of the daily quantity of walking or active travel is likely to be of the order of 1.5 times the mean value [17,20,40,62]; and systematic reviews suggest a realistic mean increase in active travel time attributable to an effective intervention of the order of 15 to 30 min/week [22,24]. We therefore aimed to achieve a sample size of 394 in each group at follow-up (788 in total), which would provide 80% power to detect a standardised mean difference (d) between the intervention and control groups of 0.20 using a two-sample t-test (a = 0.05, two-sided). This equates to a mean increase in active commuting time of 2 min/day, assuming a standard deviation of 10 min/day. Such an effect size would, appropriately, be smaller than the pooled estimate reported in the Cochrane review of controlled trials to promote physical activity (d = 0.28 [7]) but would still be large enough to be worth detecting. Quantitative analysis Questionnaire and objective measurement data will be checked and cleaned using a combination of range and