Krølner et al. BMC Public Health 2012, 12:191 http://www.biomedcentral.com/1471-2458/12/191 building at the intervention schools at 2nd follow-up (e.g. initiation of FV programmes, increased sale of FV in schools, increased focus on school food policies or limiting pupils’ access to unhealthy food at school). Unintended adverse effects: It will be evaluated whether the intervention and the increased focus on healthy eating has produced a significant increase in weight-and eating-related teasing or bullying. There is a risk that the free FV in school only replace FV eaten at other times of the day which will hinder an effect on the pupils’ total intake of FV [22]. We will therefore analyse whether fewer adolescents eat FV at home and bring FV to school from home after the intervention. As unin- tended adverse effects are difficult to predict they will also be illuminated through qualitative interviews with pupils and teachers as part of process evaluation (see Table 4). Planned statistical effect analyses The primary aim of the effect evaluation will be to assess whether our goal of a 20% increase in FV intake has been achieved. All further impacts of the intervention such as intended proximal effects (determinants) and unintended positive and negative side effects will also be subject to evaluation. Outcomes will be analysed after the principle of intention-to-treat including all pupils in the arm to which they were allocated whether or not they received (or completed) the intervention given to that arm. These results will be compared to per protocol analyses of pupils at schools that complied sufficiently with the inter- vention manual to exhibit the effect of treatment at the individual level. Descriptive analyses will be used to cal- culate the net effect of the intervention ((follow-up inta- keintervention -baseline intakeintervention) -(follow-up intakecontrol-baseline intakecontrol)). The effect of the mul- ticomponent intervention will be analysed for each and all six primary outcome measures of FV intake. To account for the cluster-design and repeated measure- ments, changes in consumption from baseline to first- and second follow-up will be analysed by multilevel mul- tivariate regression analyses. All analyses will be stratified by gender and we will examine if the intervention have differential effects on FV intake among pupils from low and high social class. Our hypothesis is that the intake of teenagers from low income families at school will benefit more from the free FV provision than their peers from high income families as they may not usually bring FV from home. In another analysis implementation degree (high/med- ium/low) will be included as we hypothesise that adoles- cents at schools/school classes who have been exposed to a large dose of the intervention will have a higher intake at 1st follow-up compared to adolescents at schools where the intervention has not been fully implemented. Page 17 of 25 In a third analysis we will try to examine if it is possible to decompose the effects of the different intervention components through mediation analysis. We will analyse the effect of the different intervention components on the determinants (proximal outcomes) they have been tailored to. One hypothesis is for example that pupils at interven- tion schools will like a larger variety of FV than the pupils at control schools at 1st follow-up due to the free FV pro- vision. In a second analytical step we will examine if change in intake is preceded by change in determinants through mediation analyses. Parent non-response at baseline and pupil and parent attrition between baseline and 1st and 2nd follow-up will be analysed by multilevel analyses using data from pupil baseline survey and national registries. Baseline data collection Pupils completed internet-based questionnaires in class and brought home questionnaires for their parents to complete. As fathers and mothers may influence their chil- dren’s food habits differently [81], both parents were invited to complete questionnaires. At all schools, trained project staff helped teachers carry out the surveys. The year 7 pupils received standardised oral instructions and a factsheet with definitions and photos of portion sizes. Dur- ing the baseline visit, observations and photo documenta- tion were made of the school food environment such as presence of canteen or fruit tuck shop, and classroom con- ditions. A pupil or teacher in each class was briefly inter- viewed about lunch habits among classmates and unwritten school rules. All principals completed an elec- tronic questionnaire on structural, physical and social characteristics of their school. Pupil, parent and principal surveys and observations will be repeated at 1st and 2nd follow-up. In January 2011, managers and coaches of clubs and teams were asked to complete internet-based baseline surveys on availability of FV and unhealthy food and exis- tence of policies before receiving Boost information material. A 1st follow-up was carried out in September/ October 2011. Response rates and characteristics of pupils at baseline As shown in Figure 2, response rates were high (> 94%) among both pupils and school principals at intervention- and control schools. A few pupil questionnaires were excluded from the baseline data file because of lack of parental consent. Only three pupil questionnaires were unserious and therefore excluded. The parental response rate was lower in both groups, but higher at intervention schools. Most of the returned parent questionnaires (60%) were completed by female caretakers. The response rates among coaches and managers of sports and youth-clubs were good (> 60%). Baseline characteristics of pupils from intervention- and control schools were compared by chi-square tests