In North America, where, for ethical reasons, randomization requires informed consent, only two such trials have been conducted. The first was in New York in the 1960s, using annual mammograms and breast examinations for the screened group.
北美地区因道德原因开展随机抽样需要知情同意书，因此上述试验只进行过两次。第一次是在2 0世纪60年代的纽约，筛查组每年接受X光乳腺造影和乳房检查。