( CNN ) -- A new study released Thursday suggests mammograms might not be as effective in reducing deaths from breast cancer in women over 50 as previously thought . The study , published in the New England Journal of Medicine , analyzed data from more than 40,000 women with breast cancer . It concluded the availability of mammography screening corresponded with a reduction in the rate of death from breast cancer , `` but the screening itself accounted for only about a third of the total reduction . '' The study involved women in Norway between ages 50 and 69 . Researchers from Norway and Harvard University compared the death rates from breast cancer in four groups : one group of women from 1996 through 2005 who were living in Norwegian counties with screening ; another group in the same time period who were living in counties without screening ; and two similar groups from 1986 through 1995 . `` Among women in the nonscreening group , there was an 18 percent reduction in the rate of death from breast cancer , as compared with the preceding 10-year period , presumably as a result of increased breast-cancer awareness , improved therapy , and the use of more sensitive diagnostic tools , '' the findings state . `` Among women in the screening group , there was a 28 percent reduction in mortality from breast cancer during the same period . Thus , the relative reduction in mortality that was causally related to the screening program alone was 10 percent . '' The publication notes that in 2002 , the World Health Organization concluded that mammography screening for women between ages 50 and 69 reduced the death rate from breast cancer by 25 percent . Researchers said challenges in measuring the effectiveness of mammography screenings include increases in breast-cancer awareness and advances in treatment .
Study : The difference in mortality related to a screening program alone was 10 percent . The study says advances in awareness and treatments have likely improved survival . Comment: