title: Sample size determination for a trial of breast cancer screening under age 50: population versus case mortality approach.
abstract: OBJECTIVE
  To determine the efficacy of screening for breast cancer in women aged 40 to 49.
  
  
  METHODS
  A large number of women will have to be enrolled to ensure adequate statistical power. Attention will be restricted to the deaths from breast cancer that occur among comparable groups of patients with breast cancer rather than to an analysis in which all women entering into the trial are considered at risk of dying from breast cancer (population mortality analysis). As the new trial is likely to be conducted in regions where screening has been established for all women from age 50 onward, comparability may be reached by including the breast cancers detected at the routine screening examination at age 50.
  
  
  CONCLUSIONS
  When comparability of cases can be reached, such a case mortality analysis requires 15-30% fewer women to be enrolled than a population approach.
mesh_terms: D000328:Adult; D000367:Age Factors; D001943:Breast Neoplasms / Q000175:diagnosis / Q000453:epidemiology* / Q000401:mortality / Q000517:prevention & control*; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008875:Middle Aged; D011336:Probability; D015203:Reproducibility of Results; D012307:Risk Factors; D018401:Sample Size; D016019:Survival Analysis; D013997:Time Factors