title: Statistical analyses in Swedish randomised trials on mammography screening and in other randomised trials on cancer screening: a systematic review.
abstract: OBJECTIVE
  We compared calculations of relative risks of cancer death in Swedish mammography trials and in other cancer screening trials.
  
  
  METHODS
  Men and women from 30 to 74 years of age.
  
  
  METHODS
  Randomised trials on cancer screening.
  
  
  METHODS
  For each trial, we identified the intervention period, when screening was offered to screening groups and not to control groups, and the post-intervention period, when screening (or absence of screening) was the same in screening and control groups. We then examined which cancer deaths had been used for the computation of relative risk of cancer death.
  
  
  METHODS
  Relative risk of cancer death.
  
  
  RESULTS
  In 17 non-breast screening trials, deaths due to cancers diagnosed during the intervention and post-intervention periods were used for relative risk calculations. In the five Swedish trials, relative risk calculations used deaths due to breast cancers found during intervention periods, but deaths due to breast cancer found at first screening of control groups were added to these groups. After reallocation of the added breast cancer deaths to post-intervention periods of control groups, relative risks of 0.86 (0.76; 0.97) were obtained for cancers found during intervention periods and 0.83 (0.71; 0.97) for cancers found during post-intervention periods, indicating constant reduction in the risk of breast cancer death during follow-up, irrespective of screening.
  
  
  CONCLUSIONS
  The use of unconventional statistical methods in Swedish trials has led to overestimation of risk reduction in breast cancer death attributable to mammography screening. The constant risk reduction observed in screening groups was probably due to the trial design that optimised awareness and medical management of women allocated to screening groups.
authors.affiliation: Institute of Global Public Health at iPRI, International Prevention Research Institute, University of Strathclyde, Ecully Lyon 69130, France International Prevention Research Institute (iPRI), Lyon 69006, France philippe.autier@i-pri.org.
authors.forename: Philippe
authors.initials: P
authors.lastname: Autier
authors.affiliation: Institute of Global Public Health at iPRI, International Prevention Research Institute, University of Strathclyde, Ecully Lyon 69130, France International Prevention Research Institute (iPRI), Lyon 69006, France.
authors.forename: Mathieu
authors.initials: M
authors.lastname: Boniol
authors.affiliation: International Prevention Research Institute (iPRI), Lyon 69006, France.
authors.forename: Michel
authors.initials: M
authors.lastname: Smans
authors.affiliation: Institute of Cancer Policy, Kings Health Partners Cancer Centre, London SE1 9RT, UK.
authors.forename: Richard
authors.initials: R
authors.lastname: Sullivan
authors.affiliation: Institute of Global Public Health at iPRI, International Prevention Research Institute, University of Strathclyde, Ecully Lyon 69130, France International Prevention Research Institute (iPRI), Lyon 69006, France.
authors.forename: Peter
authors.initials: P
authors.lastname: Boyle
mesh_terms: D001943:Breast Neoplasms / Q000175:diagnosis / Q000401:mortality*; D055088:Early Detection of Cancer; D005260:Female; D006801:Humans; D008297:Male; D008327:Mammography; D008403:Mass Screening / Q000379:methods*; D017063:Outcome Assessment, Health Care; D012107:Research Design; D012306:Risk; D013548:Sweden / Q000453:epidemiology
keywords: breast cancer; randomised trials; screening; statistical analyses