Challenges in assessing the implementation and effectiveness of physical activity and nutrition policy interventions as natural experiments.

The Ontario (Canada) government has instituted a policy requiring elementary schools to offer at least 20 min of daily physical activity for students in Grades 1-8 and replace non-nutritious vending machine foods with healthier choices. These policy interventions represent 'natural experiments' offering unique opportunities for conducting research and evaluation. The use of natural experiments to contribute evidence on the effectiveness of policy interventions is identified as an underused tool for public health [Tudor-Locke, C., Ainsworth, B. E. and Popkin, B. M. (2001) Active commuting to school: an overlooked source of children's physical activity? Sports Medicine, 31, 309-313; Petticrew, M., Cummins, S., Ferrell, C., Findlay, A., Higgins, C., Hoy, C. et al. (2005) Natural experiments: an underused tool for public health? Public Health, 119, 751-757]. To date, some Canadian school-based food and nutrition policies are being monitored, but their impact on child and youth obesity is unknown [Canadian Institute for Health Information. (2006) Improving the Health of Canadians: Promoting Healthy Weights, Ottawa, ON]. There are a number of challenges to the evaluation of policy interventions as natural experiments. Often, there are little or no baseline data available to use as the basis for assessing change. Government policies that result in the adoption of particular approaches across large jurisdictions, such as provinces, may result in wide variation in the design and implementation of interventions. Thus, in some cases, natural experiments may be at risk of having low potential to be adequately evaluated on key outcomes. In this paper, we discuss the context of these challenges in relation to the Ontario government school physical activity and nutrition policies.