title: Noncompliance in lifestyle intervention studies: the instrumental variable method provides insight into the bias.
abstract: OBJECTIVE
  In lifestyle intervention trials, participants of the control group often change their behavior despite the request to maintain their usual lifestyle pattern. These changes in the control group and changes in addition to the intended in the intervention group can lead to undesirable confounding effects.
  
  
  METHODS
  We address several considerations for study design to prevent noncompliance or minimize its effects. Furthermore, we demonstrate how the instrumental variable method can give insight into the extent of bias introduced by noncompliance in randomized trials, within the context of the Sex Hormones and Physical Exercise study.
  
  
  RESULTS
  Noncompliance can be prevented by measures taken in the design phase of a study, for example, limited duration of the study, clear recommendations, power calculation, intensity of the intervention, involvement of the control group, waiting-list control group, and single-consent design nested within an observational study. When nevertheless noncompliance does occur, the instrumental variable method estimates the intervention effect of treatment among the compliers.
  
  
  CONCLUSIONS
  Noncompliance can seriously affect validity of lifestyle trial results. Its occurrence should be prevented by taking measures during the design phase of a study. The instrumental variable method can give insight into confounding by noncompliance in randomized trials.
authors.affiliation: Julius Center for Health Sciences and Primary Care, University Medical Center Utrecht, STR 6.131, PO Box 85500, 3508 GA, Utrecht, The Netherlands.
authors.forename: Emmy M
authors.initials: EM
authors.lastname: Hertogh
authors.forename: A Jantine
authors.initials: AJ
authors.lastname: Schuit
authors.forename: Petra H M
authors.initials: PH
authors.lastname: Peeters
authors.forename: Evelyn M
authors.initials: EM
authors.lastname: Monninkhof
mesh_terms: D000368:Aged; D016022:Case-Control Studies; D015986:Confounding Factors, Epidemiologic; D015444:Exercise / Q000523:psychology; D005260:Female; D015438:Health Behavior; D006306:Health Surveys; D006801:Humans; D008019:Life Style; D008875:Middle Aged; D010349:Patient Compliance / Q000523:psychology / Q000706:statistics & numerical data*; D016032:Randomized Controlled Trials as Topic; D040242:Risk Reduction Behavior