Unbalanced Baseline in School-Based Interventions to Prevent Obesity: Adjustment Can Lead to Bias – a Systematic Review

BACKGROUND/AIMS: Cluster designs favor unbalanced baseline measures. The aim of the present study was to determine the frequency of unbalanced baseline BMI on school-based randomized controlled trials (RCT) aimed at obesity reduction and to evaluate the analysis strategies. We hypothesized that the adjustment of unbalanced baseline measures may explain the great discrepancy among studies. METHODS: The source of data was the Medline database content from January 1995 until May 2012. Our search strategy combined key words related to school-based interventions with such related to weight and was not limited by language. The participants’ ages were restricted to 6-18 years. RESULTS: We identified 146 school-based studies on obesity prevention (or overweight or excessive weight change). Of the 146 studies, 36 were retained for the analysis after excluding reviews, feasibility studies, other outcomes, and repeated publications. 13 (35%) of the reviewed studies had statistically significant (p < 0.05) unbalanced measures of BMI at baseline. 11 studies with BMI balanced at baseline adjusted for the baseline BMI, whereas no baseline adjustment was applied to the 5 unbalanced studies. CONCLUSION: Adjustment for the baseline BMI is frequently done in cluster randomized studies, and there is no standardization for this procedure. Thus, procedures that disentangle the effects of group, time and changes in time, such as mixed effects models, should be used as standard methods in school-based studies on the prevention of weight gain.