title: Generalizing results of randomized trials to clinical practice: reliability and cautions.
abstract: BACKGROUND
  Well designed randomized controlled trials provide reliable evidence of treatment effects, but there is no consensus on how best to apply these results to clinical practice. The main concerns are that populations enrolled in trials are more selected than those treated in a clinical setting, and whether the treatment effects observed in trials will also be observed in clinical practice.
  
  
  METHODS
  An informal literature review was undertaken to find studies analysing the issue of generalizing trial results (external validity) to clinical practice.
  
  
  RESULTS
  Most of the studies focused on differences in patients characteristics (age, gender, severity of disease, concomitant treatments and so on) between the clinical trial population and a 'real world' clinical population. None provided good evidence of a reduction in the treatment effect in the trial compared to what might happen in clinical practice for simple pharmacological treatments. However complex treatments like surgery or percutaneous interventional procedures, had a greater potential for variation. Extrapolating treatments to different health care settings from the trial can result in important variations in treatment effects.
  
  
  CONCLUSIONS
  Complex therapies need careful consideration before they can be applied routinely from trials into practice, and applying results from one health care environment to a different one should be carried out with caution. Generalizing results from well conducted trials to clinical practice can mostly be carried out with confidence, especially for simple therapies with good evidence of benefit.
authors.affiliation: Clinical Trials and Evaluation Unit, Royal Brompton Hospital, London, UK. m.flather@cteu.org
authors.forename: Marcus
authors.initials: M
authors.lastname: Flather
authors.forename: Nicola
authors.initials: N
authors.lastname: Delahunty
authors.forename: Julian
authors.initials: J
authors.lastname: Collinson
mesh_terms: D006801:Humans; D010349:Patient Compliance; D018579:Patient Selection; D016032:Randomized Controlled Trials as Topic / Q000379:methods* / Q000706:statistics & numerical data; D015203:Reproducibility of Results; D012107:Research Design