title: Mammographic screening: case-control studies.
abstract: BACKGROUND
  The case-control design can be used to evaluate the benefit of cancer screening programmes.
  
  
  METHODS
  This paper outlines the main methodological features of the case-control design in this context, and indicates some potential biases. It also reviews the existing case-control literature on mammographic screening.
  
  
  RESULTS
  Case-control studies consistently indicate a reduction of approximately 50% in breast cancer mortality associated with mammography. This result indicates greater benefit than shown in randomised trials; however, one should recognise that trials indicate effectiveness whereas case-control studies indicate efficacy. The two types of evidence are broadly compatible when one allows for screening non-compliance and contamination in the randomised trials.
  
  
  CONCLUSIONS
  The case-control evidence supports and is consistent with the findings of randomised trials of mammography. Effectiveness estimates from trials indicate the benefit of screening to the population as a whole, and are pertinent to the public policy debate as to the value of offering screening. In contrast, case-control studies indicate benefit to actual screening participants. As such, case-control estimates of efficacy are appropriate for individual decision-making by women about their use of mammography when it is potentially available to them.
authors.affiliation: Clinical Epidemiology and Biostatistics, McMaster University, Health Sciences Centre, Hamilton, Ontario, Canada. walter@mcmaster.ca
authors.forename: S D
authors.initials: SD
authors.lastname: Walter
mesh_terms: D000328:Adult; D017677:Age Distribution; D000368:Aged; D001943:Breast Neoplasms / Q000401:mortality* / Q000517:prevention & control*; D016022:Case-Control Studies; D002986:Clinical Trials as Topic; D005260:Female; D006801:Humans; D008327:Mammography / Q000379:methods*; D008403:Mass Screening / Q000379:methods*; D008875:Middle Aged; D015995:Prevalence; D018570:Risk Assessment; D012680:Sensitivity and Specificity; D013534:Survival