According to our descriptive epidemiology analyses, in women 40 to 69 years of age, the introduction of MMS in nine Swedish counties was associated with a 16% reduction of breast cancer mortality.
Using an incidence-based mortality method, the Swedish Organised Service Screening Evaluation Group (SOSSEG) conducted an evaluation based on comparison between women attending and not attending screening. Women not attending screening may have a less healthy lifestyle, to present with more advanced cancer and to have higher breast cancer mortality than women attending screening. In principle, failure to take into account the greater mortality of women not attending screening may lead to unbalanced comparison, and overestimation of the reduction of mortality to be expected from screening.
A companion paper to (13) using data from the same 9 counties using Poisson regression models assessed the risk of death from breast cancer in screened compared with unscreened women 
. A reduction of 42% (95% CI: 38 to 47%) in breast cancer mortality was found for screened women, after adjusting for (i) changes in breast cancer fatality rate due to improving treatments, (ii) for lead time–corrected follow-up time, and (iii) and for the temporal trend of change in incidence among non screened women before and after start of screening. The reduction was 39% (RR
0.61; 95% CI: 0.55–0.68) after further adjustment for self-selection.
The screening coverage in counties included in the SOSSEG study was 78% 
, and therefore the expected reduction in mortality for all women 50 to 69 years of age was approximately (39% * 0.78)
30%. If we further correct for the self-selection effect (women attending screening contribute slightly less than their share of deaths), this decreases by a fraction 39/42 to 28%. Our estimate of 16% should be compared to this.
The reasons for the substantial discrepancy between our estimate and those of the Swedish group 
follow from differences in model assumptions, model parameters, adjustment procedures and possibly data. The Swedish group modelled the risk of breast cancer death associated with being screened, whereas we modelled breast cancer mortality rates of the entire population eligible for screening.
One difference in approach is the correction for the change in incidence after and before the start of screening. This was done explicitly by the Swedish Group 
, which could lead to an over-estimate of the benefit of screening if there is screening-related over-diagnosis. There is relatively strong evidence that breast cancer incidence as much increased after screening introduction, which has lead to recognize overdiagnosis as being a major side effect of screening 
. Our model seeks to distinguish temporal effects on mortality (β in our model) from screening by virtue of the varying dates of screening introduction by county. We felt this approach was preferable since incidence was already increasing faster than mortality 
and it allows partial correction for both changes of incidence and treatment-related improvements in survival. Furthermore, in Sweden, breast mortality stabilized and started to decrease well before mammography screening programmes existed (see ).
Differences in data used by studies could be another possible reason for the difference in results. Breast cancer mortality data for Sweden in NordCan database are similar to those present in the WHO mortality database. We could compare numbers of breast cancer deaths in our and in the SOSSEG study (14) for the same period of time and age range for six counties (). On average, death counts used in the SOSSEG study 
were 12% lower than that from the NordCan database. We are not aware of changes in death certification process in Sweden after 1980 likely to explain this discrepancy.
Number of breast cancer deaths in women 40 to 69 years old in six Swedish counties according to two sources of data (3; 15).
It remains also to determine if the reduction in breast cancer mortality we found after the start of screening was due to mammography screening itself, or to generalization of efficient cancer treatment that took place end of the 1980s, or to greater breast awareness and improved management of breast cancer patients induced by the screening programme (e.g., because of reorganization of clinical work or greater access to efficient treatments). The influence of treatment and of awareness in mortality reductions has been stressed by model approaches showing the key role of treatments 
. In addition, in many high income countries, mortality reductions started well before or around the year screening started and these reductions were mainly observed in women less than 50 years of age even in countries where screening of young women is rare 
. Our model attempts to correct for log-linear temporal trends in mortality, whether due to treatment or risk-factor induced changes in incidence. However it will not effectively capture step changes correlated to screening as would be expected from the above factors. This question has been addressed by the study by Kalager et. al.
of mammographic screening in Norway (20), by comparing both screened and unscreened women to historic controls. They concluded that two-thirds of the reduction in mortality was likely due to non-screening factors, with screening itself generating a 10% (95% CI −4 to 24%) reduction in mortality. We would echo their conclusion: that the benefit of mammographic screening in isolation is modest, and that the larger estimates of benefit may be due to improved integration of treatment associated with screening.