title: Clinical trials: relevance of correlation between treatment modalities.
abstract: BACKGROUND
  Trials that do not allow rejection of the null hypothesis of no treatment effect may have had an inappropriate design. Trials are virtually never assessed for correlation between responses to different treatment modalities.
  
  
  METHODS
  Using a model, a simple test and several published studies we studied the influence of correlation levels on the statistical power of clinical trials.
  
  
  RESULTS
  The level of correlation between treatment modalities is a major determinant of the power of both self-controlled and parallel group clinical trials.
  
  
  CONCLUSIONS
  It is extremely relevant to assess correlation levels between treatment modalities of a trial a priori. With a presumably negative correlation a self-controlled design is likely to lack power. With a presumably positive correlation a parallel group design is likely to do so. Both designs are suitable for approximately zero correlations (e.g. comparison vs placebo).
authors.affiliation: Department of Medicine, Merwede Hospital Dordrecht, Netherlands.
authors.forename: T J
authors.initials: TJ
authors.lastname: Cleophas
mesh_terms: D002986:Clinical Trials as Topic; D003131:Combined Modality Therapy; D015233:Models, Statistical; D010919:Placebos; D013223:Statistics as Topic
chemical_list: D010919:Placebos