Test the hypothesis that the IRR comparing period A to period B is the same as from period C to period D?

One issue is that there may be time trends in breast cancer mortality that have nothing to do with screening. To control for this, a non-screening time period was identified (period C) during 1996€“2005 in each region during which breast cancer screening was not introduced (in the above region it would be from 1996€“2000). In addition, a time period 10 years prior to this non-screening period was identified during 1986€“1995 (period D) (in the above region from 1986 1990). The breast cancer mortality exposure in these two periods was as follows: 

Table 14.40: Comparison of breast cancer mortality between period C and period D

 

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# person-years # deaths Period C 1,866,741 (non-screening period from 1996–2005) (experience 10 years prior to period C) 396 Period D 1,898,989 494