Data

CRS holds information on current and historical addresses for residents in Denmark since 1968. From the CRS, we identified the target population of the Copenhagen municipality screening programme from 1 April 1991 to 12 July 2012, covering 10 biennial invitation rounds. From invitation rounds 1–8, the Copenhagen screening programme kept a mammography register with the personal identification number (including information on and date of birth), date of invitation, dates of reminders, date of screening, outcome of screening, and (where relevant) dates and outcome of assessment and surgery. The register also included information on women who had notified the programme that they did not want to be invited, and the register included codes for ineligibility. From invitation round 9 onwards, the Copenhagen programme became incorporated into the larger programme for the Capital Region of Denmark. The structure of the mammography screening register for the Capital Region differs from the old register from Copenhagen. However, it is possible from this new register to obtain the same information as before, apart from ineligibility, for which only the latest registration was kept in the new register. We linked the data retrieved from the CRS and the screening registers, which provided us with a complete invitation history of all targeted women. The use of opportunistic screening has always been very limited in Denmark.⁶