Invited Commentary Implementation evaluation of school-based obesity prevention programmes in youth; how, what and why?

In an ideal world, one combats public health problems with theoryand evidence-based programmes. In the real world, evidence-based programmes are often lacking and programmes that are developed and implemented are mainly practice-based. In the last few decades, governmental and other funding agencies have prioritized the development and evaluation of evidence-based obesity prevention programmes to combat the major public health problem of childhood obesity. Consequently, a large variety of healthy nutrition and physical activity promotion programmes targeting youth have been developed and evaluated in more or less controlled and real-world settings. Unfortunately, the real-world effectiveness of many programmes is disappointing, especially in the long term. This lack of effectiveness could be because the programme was not effective in itself or because it was not implemented as intended. It is therefore of obvious importance to evaluate if and to what extent a programme was implemented as intended. Implementation evaluation research can provide insight into the dynamic nature of implementation processes and key factors that are expected to be critical for achieving effectiveness of overweight and obesity prevention programmes during implementation. Three important generic implementation research questions in the context of programme evaluation are: (i) how to promote implementation as intended; (ii) what happens during implementation; and (iii) why did my programme (not) work? In the present invited commentary we discuss these three questions, enriched by our experiences with the school-based obesity prevention programme DOiT (Dutch Obesity Intervention in Teenagers).