Authors' reply

BMJ 1996; 312 doi: https://doi.org/10.1136/bmj.312.7032.709 (Published 16 March 1996) Cite this as: BMJ 1996;312:709
  1. Carol Parkes,
  2. Nicholas J Wald,
  3. Philip Murphy,
  4. Lynne George,
  5. Hilary C Watt,
  6. Roger Kirby,
  7. Paul Knekt,
  8. K J Helzlsouer,
  9. J Tuomilehto
  1. Lecturer Professor Computer programmer Laboratory research coordinator Statistician Cancer Research Campaign Cancer Screening Group, Department of Environmental and Preventive Medicine, Wolfson Institute of Preventive Medicine, Medical College of St Bartholomew's Hospital, London EC1M 6BQ
  2. Consultant urologist Department of Urology, St Bartholomew's Hospital, London EC1A 7BE
  3. Laboratory director Department of Health and Disability, National Public Health Institute and Social Insurance Institution, Research and Development Unit, Helsinki, Finland
  4. Associate professor Department of Epidemiology, Johns Hopkins University, School of Hygiene and Public Health, Training Center for Public Health Research, Washington County Health Department, MD 21742-2067, USA
  5. Research professor Department of Epidemiology and Health Promotion, National Public Health Institute, Helsinki, Finland

    EDITOR,—Paul Warren suggests that we should estimate the predictive value of measurement of prostate specific antigen direct from our data. The predictive value of a positive result depends on the prevalence of the disease. Our population included a high proportion of men under 60 (median age of those with the disease 57), so an estimate of the positive predictive value in our subjects would be lower than one based on older men, in whom the disease is more common. If screening were shown …

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