title: Pragmatic controlled clinical trials in primary care: the struggle between external and internal validity.
abstract: BACKGROUND
  Controlled clinical trials of health care interventions are either explanatory or pragmatic. Explanatory trials test whether an intervention is efficacious; that is, whether it can have a beneficial effect in an ideal situation. Pragmatic trials measure effectiveness; they measure the degree of beneficial effect in real clinical practice. In pragmatic trials, a balance between external validity (generalizability of the results) and internal validity (reliability or accuracy of the results) needs to be achieved. The explanatory trial seeks to maximize the internal validity by assuring rigorous control of all variables other than the intervention. The pragmatic trial seeks to maximize external validity to ensure that the results can be generalized. However the danger of pragmatic trials is that internal validity may be overly compromised in the effort to ensure generalizability. We are conducting two pragmatic randomized controlled trials on interventions in the management of hypertension in primary care. We describe the design of the trials and the steps taken to deal with the competing demands of external and internal validity.
  
  
  CONCLUSIONS
  External validity is maximized by having few exclusion criteria and by allowing flexibility in the interpretation of the intervention and in management decisions. Internal validity is maximized by decreasing contamination bias through cluster randomization, and decreasing observer and assessment bias, in these non-blinded trials, through baseline data collection prior to randomization, automating the outcomes assessment with 24 hour ambulatory blood pressure monitors, and blinding the data analysis.
  
  
  CONCLUSIONS
  Clinical trials conducted in community practices present investigators with difficult methodological choices related to maintaining a balance between internal validity (reliability of the results) and external validity (generalizability). The attempt to achieve methodological purity can result in clinically meaningless results, while attempting to achieve full generalizability can result in invalid and unreliable results. Achieving a creative tension between the two is crucial.
authors.affiliation: Centre for Studies in Primary Care, Queen's University, Kingston K7L 5E9, Canada. godwinm@post.queensu.ca
authors.forename: Marshall
authors.initials: M
authors.lastname: Godwin
authors.forename: Lucia
authors.initials: L
authors.lastname: Ruhland
authors.forename: Ian
authors.initials: I
authors.lastname: Casson
authors.forename: Susan
authors.initials: S
authors.lastname: MacDonald
authors.forename: Dianne
authors.initials: D
authors.lastname: Delva
authors.forename: Richard
authors.initials: R
authors.lastname: Birtwhistle
authors.forename: Miu
authors.initials: M
authors.lastname: Lam
authors.forename: Rachelle
authors.initials: R
authors.lastname: Seguin
mesh_terms: D001794:Blood Pressure; D018660:Blood Pressure Monitoring, Ambulatory; D005194:Family Practice; D005500:Follow-Up Studies; D006801:Humans; D006973:Hypertension / Q000175:diagnosis / Q000628:therapy; D010349:Patient Compliance; D018579:Patient Selection; D016032:Randomized Controlled Trials as Topic / Q000379:methods*; D015203:Reproducibility of Results; D012107:Research Design; D016896:Treatment Outcome