These programs improve the food environment in schools through changes such as removing soda and energy dense food from cafeterias and vending machines, improving the nutrition of school meals, promoting water consumption, and encouraging students to bring healthier food from home. Twelve of the 14 programs included in this review also included increased opportunities for physical activity during the school day.

 

Meta-analysis is a statistical method to combine the results from separate studies on a program, policy, or topic to estimate its effect on an outcome. WSIPP systematically evaluates all credible evaluations we can locate on each topic. The outcomes measured are the program impacts measured in the research literature (for example, impacts on crime or educational attainment). Treatment N represents the total number of individuals or units in the treatment group across the included studies.

An effect size (ES) is a standard metric that summarizes the degree to which a program or policy affects a measured outcome. If the effect size is positive, the outcome increases. If the effect size is negative, the outcome decreases. See Estimating Program Effects Using Effect Sizes for additional information on how we estimate effect sizes.

The effect size may be adjusted from the unadjusted effect size estimated in the meta-analysis. Historically, WSIPP adjusted effect sizes to some programs based on the methodological characteristics of the study. For programs reviewed in 2024 or later, we do not make additional adjustments, and we use the unadjusted effect size whenever we run a benefit-cost analysis.

Research shows the magnitude of effects may change over time. For those effect sizes, we estimate outcome-based adjustments, which we apply between the first time ES is estimated and the second time ES is estimated. More details about these adjustments can be found in our Technical Documentation.

Meta-Analysis of Program Effects
Outcomes measured	Treatment age	Primary or secondary participant	No. of effect sizes	Treatment N	Effect sizes (ES) and standard errors (SE)			Unadjusted effect size (random effects model)
					ES	SE	Age	ES	p-value
Obesity Obese based on clinical guidelines for adults (body mass index of 30 or higher) or children (body mass index at or above the 95th percentile for children of the same age and sex), as appropriate.	9	Primary	14	12400	-0.106	0.039	10	-0.106	0.007

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Marcus, C., Nyberg, G., Nordenfelt, A., Karpmyr, M., Kowalski, J., & Ekelund, U. (2009). A 4-year, cluster-randomized, controlled childhood obesity prevention study: STOPP. International Journal of Obesity, 33(4), 408-417.

Muckelbauer, R., Libuda, L., Clausen, K., Reinehr, T., & Kersting, M. (2009). A simple dietary intervention in the school setting decreased incidence of overweight in children. Obesity Facts, 2(5), 282-285.

Sahota, P., Rudolf, M., Dixey, R., Hill, A., Barth, J., & Cade, J. (2001). Randomised controlled trial of primary school based intervention to reduce risk factors for obesity. British Medical Journal, 323(7320), 1029-1032.

Sallis, J.F., Mckenzie, T.L., Conway, T.L., Elder, J.P., Prochaska, J.J., Brown, M., . . . Alcaraz, J.E. (2003). Environmental interventions for eating and physical activity - A randomized controlled trial in middle schools. American Journal of Preventive Medicine, 24(3), 209-217.

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