title: Testing for baseline differences in randomized controlled trials: an unhealthy research behavior that is hard to eradicate.
abstract: BACKGROUND
  According to the CONSORT statement, significance testing of baseline differences in randomized controlled trials should not be performed. In fact, this practice has been discouraged by numerous authors throughout the last forty years. During that time span, reporting of baseline differences has substantially decreased in the leading general medical journals. Our own experience in the field of nutrition behavior research however, is that co-authors, reviewers and even editors are still very persistent in their demand for these tests. The aim of this paper is therefore to negate this demand by providing clear evidence as to why testing for baseline differences between intervention groups statistically is superfluous and why such results should not be published.
  
  
  CONCLUSIONS
  Testing for baseline differences is often propagated because of the belief that it shows whether randomization was successful and it identifies real or important differences between treatment arms that should be accounted for in the statistical analyses. Especially the latter argument is flawed, because it ignores the fact that the prognostic strength of a variable is also important when the interest is in adjustment for confounding. In addition, including prognostic variables as covariates can increase the precision of the effect estimate. This means that choosing covariates based on significance tests for baseline differences might lead to omissions of important covariates and, less importantly, to inclusion of irrelevant covariates in the analysis. We used data from four supermarket trials on the effects of pricing strategies on fruit and vegetables purchases, to show that results from fully adjusted analyses sometimes do appreciably differ from results from analyses adjusted for significant baseline differences only. We propose to adjust for known or anticipated important prognostic variables. These could or should be pre-specified in trial protocols. Subsequently, authors should report results from the fully adjusted as well as crude analyses, especially for dichotomous and time to event data. Based on our arguments, which were illustrated by our findings, we propose that journals in and outside the field of nutrition behavior actively adopt the CONSORT 2010 statement on this topic by not publishing significance tests for baseline differences anymore.
authors.affiliation: Department of Health Sciences and the EMGO Institute for Health and Care Research, Faculty of Earth and Life Sciences, VU University Amsterdam, De Boelelaan 1085, 1081 HV, Amsterdam, The Netherlands. m.r.de.boer@vu.nl.
authors.forename: Michiel R
authors.initials: MR
authors.lastname: de Boer
authors.affiliation: National Institute for Health Innovation, School of Population Health, The University of Auckland, Tamaki Campus, Private Bag 92019, Auckland Mail Centre, Auckland, 1142, New Zealand. w.waterlander@auckland.ac.nz.
authors.forename: Wilma E
authors.initials: WE
authors.lastname: Waterlander
authors.affiliation: Department of Health Sciences and the EMGO Institute for Health and Care Research, Faculty of Earth and Life Sciences, VU University Amsterdam, De Boelelaan 1085, 1081 HV, Amsterdam, The Netherlands. l.d.j.kuijper@vu.nl.
authors.forename: Lothar D J
authors.initials: LD
authors.lastname: Kuijper
authors.affiliation: Department of Health Sciences and the EMGO Institute for Health and Care Research, Faculty of Earth and Life Sciences, VU University Amsterdam, De Boelelaan 1085, 1081 HV, Amsterdam, The Netherlands. i.h.m.steenhuis@vu.nl.
authors.forename: Ingrid H M
authors.initials: IH
authors.lastname: Steenhuis
authors.affiliation: Department of Epidemiology and Biostatistics, VU University Medical Centre, de Boelelaan 1118, Amsterdam, 1081 HV, The Netherlands. jwr.twisk@vumc.nl.
authors.forename: Jos W R
authors.initials: JW
authors.lastname: Twisk
mesh_terms: D006801:Humans; D008297:Male; D011643:Publishing; D016032:Randomized Controlled Trials as Topic; D012107:Research Design