Quote below from the breast cancer study, if I am reading it right there was a higher death rate in the control group before the study period, so this already suggests a confounding factor. In addition to that it would be interesting to actually know how many diagnosis and treatments there were in both the control and study group to determine if the screening was actually effective or not. I also wonder about the definition of deaths and how many may have been classified as another cause some time after treatment and if it was related possibly.
By using the definition of refined mortality, there were 607 breast cancer deaths during 4.8 million person-years in the study group and 846 breast cancer deaths during 6.3 million person-years in the control group during the reference period (from 1970 to 1985; ie, before the start of service screening), resulting in an RR of 0.94 (95% confidence interval [CI], 0.85-1.05). For the study period (from 1986 to 2005), there were 803 breast cancer deaths during 7.3 million person-years in the study group (Table 2) and 1238 breast cancer deaths during 8.8 million person-years in the control group (Table 3), resulting in an estimated crude RR of 0.79 (95% CI, 0.72-0.86), (Table 4). The crude cumulative breast cancer mortality per 100,000 person-years is illustrated in Figure 2. The curves start to diverge after 3 years and continue to diverge throughout follow-up.