The trials that have shown a significant decrease in mortality are usually older, and other factors have compromised the results. For example, the Swedish two-county trial was randomized by clusters of population groups, and the comparability between the women in the screened and control groups was not confirmed.
显示筛查能降低死亡率的试验往往为时较早，而且结果也受到其他因素的影响。比方说，瑞典两县试验随机抽选不同的人群，无法确认筛查组和控制组能得出可比的结果。此外，该试验进行时还没有免费的现代乳腺癌治疗技术。