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The Diabetes Educator

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Health Education & Behavior, Vol. 31, No. 6, 756-774 (2004)
DOI: 10.1177/1090198104263406

Assessing Intervention Effects in a School-Based Nutrition Intervention Trial: Which Analytic Model Is Most Powerful?

Jessica B. Janega, PhD

Department of Psychology, University of Memphis

David M. Murray, PhD

Department of Psychology, University of Memphis

Sherri P. Varnell, MS, PhD

Northrop-Grumman Mission Systems

Jonathan L. Blitstein, MS

Department of Psychology, University of Memphis

Amanda S. Birnbaum, PhD, MPH

Division of Epidemiology, School of Public Health, University of Minnesota

Leslie A. Lytle, PhD

Division of Epidemiology, School of Public Health, University of Minnesota

This article compares four mixed-model analyses valid for group-randomized trials (GRTs) involving a nested cohort design with a single pretest and posttest. This study makes estimates of intraclass correlations (ICCs) available to investigators planning GRTs addressing dietary outcomes. It also provides formulae demonstrating the potential benefits to the standard error of the intervention effect ({sigma}{Delta}) from adjustments for both fixed and time-varying covariates and correlations over time. These estimates will allow other researchers using these variables to plan their studies by estimating a priori detectable differences and sample size requirements for any of the four analytic options. These methods are demonstrated using data from the Teens Eating for Energy and Nutrition at School study. Mixed-model analyses of covariance proved to be the most powerful analysis in that data set. The formulae may be applied to any dependent variable in any GRT given corresponding information for those variables on the parameters that define the formulae.

Key Words: group-randomized trial • analytic methods • power


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