Estimate the incidence rate ratio (IRR) comparing mortality incidence between period A and period B, and provide a 95% CI about this estimate?

Cancer

A study was conducted concerning the effect of screening mammography on breast cancer mortality in Norway (Kalager et al. [15]). A recommendation was made starting in 1996 to offer screening mammography to women ages 50€“69. The 19 counties in Norway were grouped into 6 regions, and the screening project was introduced in a staggered fashion starting from 1996 until 2005. Thus, each region had a time period from part of 1996 to 2005 during which screening was offered (period A). In addition, historical information was used to estimate breast cancer mortality from 1986€“1995 prior to when screening was introduced (period B). Thus, if screening was in place from 2001€“2005 in a region, the historical exposure from 1991€“1995 was considered in the same region. The results were as follows: 

Table 14.39 Comparison of breast cancer mortality between period A and period B

 

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# deaths # person-years Period A 2,337,323 (screening period from 1996–2005) 2,197,469 (experience 10 years 423 Period B 555 prior to period A)