title: In a randomized controlled trial, missing data led to biased results regarding anxiety.
abstract: OBJECTIVE
  Randomization does not protect against bias due to missing observations. In addition, different reasons for missing observations may lead to different invalid results. The purpose of this study was to illustrate how randomized intervention studies can be threatened by bias due to missing observations because of death or nonresponse.
  
  
  METHODS
  A randomized clinical trial of the effect of psychosocial intervention on well-being after an operation for colorectal cancer was conducted in Denmark. Patients were interviewed 3, 6, 12, and 24 months after discharge from hospital.
  
  
  RESULTS
  We found that the probability of nonresponse decreased with increasing anxiety score in the intervention group, but it increased with increasing anxiety score in the control group. This could lead to severe bias in an analysis of the effect of intervention on anxiety. Low physical functioning and low global health status and quality of life were related to an increased probability of dying before the next follow-up, and this association could explain the associations between anxiety and depression, respectively, and the probability of dying observed in crude analyses.
  
  
  CONCLUSIONS
  Our study emphasizes the importance of performing specific missing data analyses in any study of well-being variables.
authors.affiliation: Institute of Cancer Epidemiology, Danish Cancer Society, Strandboulevarden 49, 2100 Copenhagen, Denmark. ross@cancer.dk
authors.forename: Lone
authors.initials: L
authors.lastname: Ross
authors.forename: Birthe Lykke
authors.initials: BL
authors.lastname: Thomsen
authors.forename: Ellen Helle
authors.initials: EH
authors.lastname: Boesen
authors.forename: Christoffer
authors.initials: C
authors.lastname: Johansen
mesh_terms: D000368:Aged; D001007:Anxiety / Q000401:mortality / Q000628:therapy*; D015982:Bias; D015179:Colorectal Neoplasms / Q000601:surgery; D003627:Data Interpretation, Statistical; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D010352:Patient Dropouts; D011184:Postoperative Period; D016032:Randomized Controlled Trials as Topic; D012944:Social Support; D016896:Treatment Outcome; D016312:Treatment Refusal