title: Comparisons against baseline within randomised groups are often used and can be highly misleading.
abstract: BACKGROUND
  In randomised trials, rather than comparing randomised groups directly some researchers carry out a significance test comparing a baseline with a final measurement separately in each group.
  
  
  METHODS
  We give several examples where this has been done. We use simulation to demonstrate that the procedure is invalid and also show this algebraically.
  
  
  RESULTS
  This approach is biased and invalid, producing conclusions which are, potentially, highly misleading. The actual alpha level of this procedure can be as high as 0.50 for two groups and 0.75 for three.
  
  
  CONCLUSIONS
  Randomised groups should be compared directly by two-sample methods and separate tests against baseline are highly misleading.
authors.affiliation: University of York, Heslington, York, YO10 5DD, UK. martin.bland@york.ac.uk
authors.forename: J Martin
authors.initials: JM
authors.lastname: Bland
authors.forename: Douglas G
authors.initials: DG
authors.lastname: Altman
mesh_terms: D000375:Aging / Q000187:drug effects; D001416:Back Pain / Q000628:therapy; D003198:Computer Simulation; D005260:Female; D006801:Humans; D008595:Menorrhagia / Q000188:drug therapy; D011336:Probability; D016032:Randomized Controlled Trials as Topic / Q000379:methods*