Summary of benefits and harms when 1,000 women are screened every second years for 20 years starting at age 50. Number of women with false positive mammograms and false positive biopsies are based on a review . Number of interval cancers are based on reported number of interval cancer in the National Health Service breast screening programme . The numbers of overdiagnosed and prevented breast cancer deaths are estimated based on 31% overdiagnosis  and 13 to 17% reduction in mortality from breast cancer . These relative numbers are applied to the observed incidence of invasive breast cancer (women aged 50 to 69 years) and mortality (women aged 55 to 74 years) in the UK in 2007 ; this resulted in 15 overdiagnosed women and 2 to 3 prevented breast cancer deaths per 1,000 women. No deaths are prevented overall .