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Concerning Sichieri R, Cunha DB: Obes Facts 2014;7:221-232. The Assertion that Controlling for Baseline (Pre-Randomization) Covariates in Randomized Controlled Trials Leads to Bias Is False

Peng Li,Andrew W. Brown,J. Dawson,K. Kaiser,Michelle M Bohan Brown,S. Keith,J. Oakes,D. Allison

Published 2015 in Obesity Facts

ABSTRACT

We read with interest the article ‘Unbalanced Baseline in School-Based Interventions to Prevent Obesity: Adjustment Can Lead to Bias – a Systematic Review’ [1] , hereafter ‘ the article ’. We agree with the authors that more rigor is needed in research on obesity treatment and prevention, and in the design, analysis, and reporting of cluster randomized controlled trials (cRCTs) [2] , also called group randomized trials. Unfortunately, rather than offering clarifying information, the article is based on incorrect statistical reasoning and inaccurate statements about what past publications have shown. The fundamental conclusion as stated in its title and elsewhere in the article is incorrect. For example, the statement ‘Although adjusting for the baseline values of parameters (sic, variables – Li et al.) that are highly influenced by baseline values is a standard procedure, this approach can bias the results …’ is simply untrue. Such erroneous conclusions could lead researchers to avoid legitimate power-enhancing analytic methods, and should be retracted. Adjusting for pre-randomization covariates in randomized trials does not introduce bias nor invalidate significance tests. This is known from statistical principles and requires neither simulation nor meta-analyses. By definition and design, in randomized experiments prerandomization covariates are independent of treatment assignment, with the exception of chance deviations which are accommodated in the calculation of frequentist significance tests and their associated p values. If the outcome variable (Y) is measured pre-randomization (Y 0 ) and at the end of the study (Y 1 ), then using either Y 1 or Y 0 – Y 1 as outcomes and either

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