title: Criticism of cardiovascular studies with negative results due to a negative correlation.
abstract: BACKGROUND
  Trials that do not allow rejection of the null hypothesis of no treatment effect may have had an inappropriate design. Self- controlled trials, although routinely used for the study of cardiovascular diseases, are virtually never assessed for correlation between treatment modalities.
  
  
  METHODS
  Using three models and a series of published studies as examples, the author studies the influence of correlation levels on the statistical power of self-controlled studies.
  
  
  RESULTS
  Between-subject variation as estimated by SD is largely dependent on the level of correlation, and so is the power of testing. The assessment of paired data as though they are unpaired can be used as a simple test to estimate correlation levels.
  
  
  CONCLUSIONS
  It is extremely relevant to assess correlation levels in a paired comparison a priori. With a presumably negative correlation, a self- controlled design is likely to lack power.
authors.affiliation: Department of Medicine, Merwede Hospital, Dordrecht, the Netherlands.
authors.forename: T J
authors.initials: TJ
authors.lastname: Cleophas
mesh_terms: D002318:Cardiovascular Diseases / Q000628:therapy*; D006801:Humans; D012107:Research Design; D013223:Statistics as Topic