There have been several attempts to evaluate, through observational studies, the impact of screening in the general population. But such studies, which are based on comparisons between uncontrolled groups, tend to produce biased results.
人们曾多次尝试通过观察研究来评估筛查在大众中的作用。但上述基于非受控组比较的研究往往得出的结果会有偏差，因此在治疗上无法带来实质性的不同。更重要的是，筛查组中晚期乳腺癌病例也并未出现下降——而这是认定筛查有效的必要前提。