BMC Public Health 2009, 9:207 http://www.biomedcentral.com/1471-2458/9/207 ing). Checklist SPE factors will be operationalised for measurement by modifying existing tools where these are available (e.g. walkability index), or by developing new tools where necessary. Trained observers will visit inter- vention and control areas and use a pro-forma to collect data on SPE using a systematic address-based sampling approach before and after intervention delivery. Data will be analyzed to yield a simple index for each SPE factor and changes in these over time will be compared between the intervention and control sites. Process evaluation The process evaluation will assess the implementation and receipt of the interventions and will aid interpretation of the results. In particular it can distinguish between faulty interventions and those badly delivered. It is espe- cially necessary in multisite trials where interventions with the same goals may consist of differing elements and be implemented and received in different ways [64] The evaluation of the delivery of the Well London pro- gramme will be carried out in partnership with the Lon- don Health Commission and the Big Lottery Fund and will be in line with commitments made in the Well Lon- don Strategy Document [65]. There will be both a contin- uous process evaluation conducted by the Well London Alliance and external evaluations commissioned in years three and five. The evaluation will take place at two levels, the programme level and the project/LSOA level. At the programme level the goals are to: a) document structures and processes through which the Well London programme is managed, and through which projects are delivered; b) identify critical pathways and steps in these processes; c) identify key challenges and barriers in these structures and processes and document how these were overcome, and d) identify key indicators for the GLA's performance management process. At the LSOA/project level the goals are to: a) document and describe key struc- tures and processes involved in the development and management of the Well London programme at LSOA level; b) document and describe key structures and proc- esses involved in the delivery of projects at LSOA level and the mechanisms through which these are anticipated to, and do generate outcomes; c) document outcomes among project beneficiaries; d) identify key challenges and docu- ment how these were overcome, and e) inform GLA's per- formance management, monitoring and review process on key items of data. Evaluation methods will include: a) analysis of quarterly statistics and information collected on activities delivered relative to plan, on performance and on cost; b) analysis of emerging project documentation to assess coherence with overall Well London objectives; c) interviews with individuals leading and delivering projects to both iden- tify design and delivery processes and to track how projects are anticipated to achieve impact; d) documenta- tion of structures, systems and processes for each project; e) identifying linkages with other projects and critical pathways; f) using focus groups convened as part of the annual community engagement process to explore the extent to which Well London activities are addressing local needs and priorities, and f) multimedia documenta- tion with a particular focus on critical pathways and chal- lenges. These methods will deliver an overall assessment of the coherence of activity delivered with Well London objec- tives as well as detailed project specifications and critical path analyses available to inform replication and roll out. They will also provide real-time information on chal- lenges and processes to allow critical reflection and reme- dial action and materials for marketing future health improvement and health promotion programmes utilis- ing a community development approach. Discussion There is a relative lack of evidence on public health inter- ventions evaluated through randomised controlled trials. One particular challenge with collecting this evidence relates to the difficulty in attributing changes in health to the interventions in question as well as difficulties in deal- ing with known and unknown confounders. The Well London study has addressed these problems by adopting a cluster randomised controlled trial design. It is thus well-placed to answer questions about the effectiveness of complex community-based interventions on health and wellbeing outcomes. Such CRCTs are also often criticised for taking a "black box" approach to the intervention, and the criticism is sometimes made that this approach is inappropriate for community interventions which need to be tailored to local needs. This CRCT also takes account of such considerations, given the involvement of local com- munities in the intervention delivery process. The multi- method approach to data collection also allows not just the overall effects to be identified but also the mecha- nisms through which those changes take place. Moreover the collection of a rich set of data on processes and context will allow a detailed description of the barriers and facili- tators to the success or failure of intervention. The interlinked datasets which will be produced as part of this project will ultimately allow us to assess the condi- tions which are necessary for the intervention to "work". This will eventually allow us and others to assess the extent to which the findings of the study are generalisable, and to state the limits on its generalisability. Overall, the large sample size the multidisciplinary multi- method approach and the robust design should help ensure that the study provides valuable evidence of the Page 9 of 11 (page number not for citation purposes)