title: Crossover studies are a better format for comparing equivalent treatments than parallel-group studies.
abstract: BACKGROUND
  In controlled clinical trials the new treatment may be a slight modification of the standard or be equivalent to it with the addition of a new component. In this situation there is a positive correlation between the response to the new and the standard treatment.
  
  
  OBJECTIVE
  The influence of the correlation between the treatment responses on the statistical sensitivity of testing was studied. Randomized trials of equivalent treatments in psychiatry and hypertension research were studied for their design in relation to their level of correlation.
  
  
  RESULTS
  With equivalent treatments and, thus, a positive correlation a paired analysis provides better power than an unpaired one.
  
  
  CONCLUSIONS
  Crossover studies are a better format than parallel-group studies for comparing equivalent treatments. The scientific community is technically largely unaware of the mechanisms by which correlation levels influence or lack to influence the statistical power of controlled clinical trials.
authors.affiliation: Department of Medicine, Merwede Hospital, Dordrecht, The Netherlands.
authors.forename: T J
authors.initials: TJ
authors.lastname: Cleophas
authors.forename: E M
authors.initials: EM
authors.lastname: de Vogel
mesh_terms: D000317:Adrenergic alpha-Antagonists / Q000008:administration & dosage / Q000494:pharmacology; D000318:Adrenergic beta-Agonists / Q000008:administration & dosage / Q000494:pharmacology; D000704:Analysis of Variance; D018849:Controlled Clinical Trials as Topic; D018592:Cross-Over Studies; D004311:Double-Blind Method; D006801:Humans; D012189:Retrospective Studies
chemical_list: D000317:Adrenergic alpha-Antagonists; D000318:Adrenergic beta-Agonists