The evidence

Although randomized controlled trials of the effectiveness of mammography screening have found that it reduces mortality in women over fifty, the benefit appears to be smaller and associated with greater harm in women less than 50 years of age. Most of the trials were not

Box 2.4 (continued)

designed specifically to address the issue of screening in younger women and included few such participants. Subsequent subgroup analyses did not therefore have sufficient statistical power to detect differences in outcome with reliability and may therefore be criticized (see Section 9.4.7). This lack of benefit in younger women was a source of debate and concern. In consequence a number of meta-analyses, published between 1993 and 1996, were undertaken to try to resolve this issue. However, the meta-analyses varied with respect to the trials that were included and methodology used, and consequently gave varying results. Although all but one suggested a benefit of screening in terms of the relative risk of mortality, the size and significance of the effect depended on the trials that were included and the statistical analyses that were done.

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