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Not too many studies present their data also in the form of numbers needed to treat (NNT) or harm (NNH). The reason for that is that there is a debate over the importance and value of such a mode of analysis. Recently, two major studies included a calculation of the NNT, which may serve as an opening point for discussing this issue. A long-term, Swedish cohort examined the effect of mammographic screening on breast cancer mortality during a 29-year follow-up period [1]. Over 133,000 women aged 40–74 years were randomized either into a group invited to mammographic screening or a control group receiving the usual care. The traditional presentation of results pointed at a highly significant decrease in breast cancer mortality in the invited-for-mammography group (relative risk 0.69; 95% confidence interval (CI) 0.56–0.84; [i]p[/i] < 0.0001). This translated into the figure of 414 women who had to undergo screening for 7 years in order to prevent one breast cancer death [1]. In the Women’s Health Initiative observational study [2], 25,448 postmenopausal women aged 50–79 years who underwent either bilateral salpingo-oophorectomy (BSO) or hysterectomy with ovarian conservation were followed for 7.6 years. It was found that removal of the ovaries saved one ovarian cancer in 323 women with BSO compared with those whose ovaries were left intact during hysterectomy [2].


  • Amos Pines
    Department of Medicine T, Ichilov Hospital, Tel-Aviv, Israel
  • Syd Shapiro
    Department of Public Health and Family Medicine, University of Cape Town, South Africa


  1. Tabár L, Vitak B, Chen TH, et al. Swedish Two-County Trial: impact of mammographic screening on breast cancer mortality during 3 decades. Radiology 2011;260:658-63.
  2. Jacoby VL, Grady D, Wactawski-Wende J, et al. Oophorectomy vs ovarian conservation with hysterectomy: cardiovascular disease, hip fracture, and cancer in the Womens Health Initiative Observational Study. Arch Intern Med 2011;171:760-8.
  3. Sanmuganathan P, Ghahramani P, Jackson P, Wallis E, Ramsay L. Aspirin for primary prevention of coronary heart disease: safety and absolute benefit related to coronary risk derived from meta-analysis of randomised trials. Heart 2001;85:26571.
  4. Riancho JA. [Number of patients to be treated and number of prevented fractures: clinical efficiency of osteoporosis treatment with diphosphonate alendronate]. Rev Clin Esp 1999;199:349-55.
  5. The National Osteoporosis Foundation. Review of the evidence for prevention, diagnosis and treatment and cost-effective analysis. Osteoporos Int 1998;8(Suppl 4):S53-5.
  6. Wisløff T, Aalen OO, Kristiansen IS. Considerable variation in NNT a study based on Monte Carlo simulations. J Clin Epidemiol 2011;64:444-50.
  7. Rossouw JE, Anderson GL, Prentice RL, et al. The Writing Group for the Womens Health Initiative Investigators. Risks and benefits of estrogen plus progestin in healthy postmenopausal women: principal results from the Womens Health Initiative Randomized Controlled trial. JAMA 2002;288:321-323.
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