Introduction: Randomized trials of breast cancer screening show that invitation to mammographic screening is associated with a significant reduction in mortality from breast cancer (1-3). Having shown the efficacy of breast cancer screening with mammography, today it is equally important to evaluate the effectiveness of mammography service screening programs. Comparison of deaths from breast cancer before and after the introduction of screening is a powerful approach but raises various problems of design, analysis, and interpretation. Failure to distinguish breast cancer deaths among women who might have benefited from screening (i.e., incident cases after the introduction of screening) from those who could not have benefited (i.e., incident cases before the introduction of screening) has resulted in very low estimates of the breast cancer mortality reduction as a result of population-based screening (4).

Other changes over an evaluation period also influence breast cancer mortality, such as changes in incidence, improveŽments in therapy, and increased awareness on the part of women to the first sign of symptoms. In the past, we have addressed the first problem by using incidence-based mortalŽity, i.e., deaths only from tumors diagnosed in the screening epoch are compared with deaths only from tumors diagnosed in the prescreening epoch (5-7). In previous evaluations, to estimate the screening effect independent of other changes, breast cancer mortality among those who did not receive screening in the screening epoch was compared with breast cancer mortality in the prescreening epoch.

Previous research on service screening in Sweden found a range of estimated mortality reductions associated with the policy of offering screening of 9% to 28% depending on the age group and region studied (8-13). Our work on incidence-based mortality indicated that women exposed to mammographic screening (i.e., women actually attending) in the screening epoch had a 40% to 50% reduced breast cancer mortality compared with unexposed women in the prescreening epoch, after adjustment for self-selection for screening (5, 14, 15). The small reduction in mortality of f15% in unscreened women in the screening epoch indicates that the majority of the 40% to 50% reduction is due to the screening and not to other changes over time (13).

The use of incidence-based mortality has been criticized, based on the assertion that use of only deaths from tumors diagnosed in each relevant epoch gives rise to length bias (16, 17). This criticism is mistaken (18) because although length bias could artificially increase the number of cases in the screening epoch, it would not affect the observed number of deaths (the numerator of the mortality rate) nor would it affect the person-years in the population as a whole (the denominaŽtor). Nevertheless, it is desirable to develop a method of analysis that uses all deaths from all tumors diagnosed throughout the total period of observation. Our companion article addresses this issue (19).

The Swedish Organised Service Screening Evaluation Group aims to draw together evidence from all parts of Sweden on the effect of organized mammographic service screening on breast cancer mortality and other end points. In this article, we report on the effect of the introduction of mammographic screening in 13 large areas within nine counties in Sweden, covering 45% of the Swedish female population, on breast cancer mortality. This analysis includes further follow-up of the six counties included in the earlier report (5), which had at least 10 years of screening activity, plus analysis of data from seven areas which recently joined the Swedish Organised Service Screening Evaluation Group collaboration. The aims of the present study are: (a) to compare mortality from breast cancer diagnosed in the prescreening and screening epochs in the 13 areas studied, providing an estimate of the effect of screening on breast cancer mortality when it is offered to the eligible female population; (b) to estimate the effect on mortality of actual exposure to screening; (c)tomake appropriate adjustments for self-selection bias, and for changes occurring contemporaneously with the introduction of screenŽing; and (d) to report on the level of screening and diagnostic activity required to produce the benefits in (a-c) above.