Krølner et al. BMC Public Health 2012, 12:191 http://www.biomedcentral.com/1471-2458/12/191 school-year and evaluated with long follow-up and in a randomised controlled design in Denmark. It is based on comprehensive reviews of both quantitative and qualita- tive studies of factors which promote teenagers’ intake of FV [3,6], it focuses on 13-year-olds, it targets both fruit and vegetable consumption, it has a strong focus on availability in all settings relevant to teenagers, it imple- ments a free FV programme in a Danish context in com- bination with educational components, it focuses on changing norms/school class environment/social practise around eating FV, and it explores ways to include sport- and youth clubs in health promotion. The survey instru- ments measure groups with low intake of FV such as boys and low social class, leaving opportunity for analysis of effect modification. The project tests innovative con- structs such as perceived appropriateness of eating FV in different occasions, settings and situations and a multidi- mensional measure of availability at home and in schools [6]. The thorough process evaluation based on multiple methods and data sources will give a comprehensive pic- ture of characteristics of the intervention and the school (context and composition) influencing implementation of educational and environmental strategies in families, schools and sports-and youth clubs. The study design enables us to examine which methods are efficient in changing specific determinants as we may link changes in determinants to whether the specific components were effectively administered. Successful recruitment of schools and high response rates among pupils have resulted in the desirable number of schools and pupils as estimated in the power calculations. We have succeeded in establishing randomisation. Analysis of baseline data showed almost equivalent groups when considering socio-demographic characteristics, primary outcomes, and FV availability at school, home and in sports-and youth clubs, a good starting point for estimating effects of the intervention at 1st and 2nd follow-up. In spite of the random allocation of schools to intervention-and control groups, significantly larger proportions of pupils at control schools had parents born in Denmark which must be taken into account in the effect analyses. Comparison with other studies Planning process As many previous intervention projects the Boost inter- vention has been designed using the systematic theory- and evidence based planning tool Intervention Mapping [51,56,57,85]. In addition we tested some new planning strategies in the study: 1) To ensure ownership of the Boost curriculum and the applicability of the teaching material in intervention schools, we gathered teachers from all intervention schools to comment on a prelimin- ary draft of the teaching material at a one-day workshop before intervention start. 2) Many countries experience a Page 20 of 25 decrease in the number of schools willing to participate in research projects which compromises the power and representativeness of the studies [86,87]. The recruitment of schools for the Boost study was thoroughly prepared and inspired by the principle of double-entry bookkeep- ing of organisational change [74]. We believe that these careful considerations about stakeholders’ interests together with the schools’ prospects of getting FV for free throughout the school year contributed to the suc- cessful recruitment process. Multi-component design Other school-based multi-component interventions have also included environmental strategies to increase access to FV in schools, a school curriculum and the involve- ment of parents [19,37,57,88]. The Boost study differs from these interventions by involving an older age group of children, by targeting FV intake separately, by serving FV for free and each day throughout the school year, and by adding a new component in youth-and sports clubs which aims at increasing teenagers’ access to FV in the leisure time setting. The Boost study has an explicit focus on accessibility in school. Accessibility is increased by means of free delivery, cutting up FV as snacks, and by allocating time for eating FV. For the school curriculum, we built on Norwegian experiences [56,89] and devel- oped a teaching material which was compatible with the official national learning objectives. Unlike most other projects [57,89], we developed tutorials which could be implemented in many subjects instead of only as part of home economics classes. The Boost computer tailoring instrument is a further development of the Pro Children computer tailoring [64]. As a new feature the children’s data entries for each time they used the program were stored in the program’s memory so the children could monitor the development in their intake over the school year to further stimulate their awareness. Implementation As in the Pro Children study and the Dutch Schoolgruiten project [57,90], the Boost intervention was designed to be implemented primarily by teachers. In the former studies the responsibility for implementation relied on one teacher alone either the main teacher or home economics teacher. In the Boost study, all teachers of year 7 shared the responsibility of implementing the curriculum and FV scheme, supported by a coordinator at the school. Some studies indicate that peer-led health intervention strategies are effective among teenagers [91-93]. In the Boost study, the pupils are widely responsible for implementing the FV break. Evaluation design The Boost study differs from other similar interventions by using a random sample of municipalities and schools including all year 7 school classes at each participating school and not only a self-selected sample of school