Krølner et al. BMC Public Health 2012, 12:191 http://www.biomedcentral.com/1471-2458/12/191 2The Danish Institute for Health Services Research (DSI), Dampfærgevej 27- 29, 2100 Copenhagen Ø, Denmark. Authors’ contributions RK is the principal investigator of the Boost study. RK conceived of and coordinated the study, contributed to its design, acquisition of data, statistical analysis, interpretation of data, and drafted the manuscript. AHC participated in collection of data, data cleaning, and statistical analyses and revised the manuscript critically. TSJ participated in the design of the study and collection of data, and revised the manuscript critically. AKA participated in the design of the study and collection of data, and revised the manuscript critically. AMC participated in the design of the study and revised the manuscript critically. PD conceived of the study, participated in its design, supervised the project group and revised the manuscript critically. 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