Study areas and time periods of the study and control groups

Time period Copenhagen Denmark outside screening regions* Pre-screening period 1981-91 Historical control group Historical national control group Screening period 1991-2001 Study group National control group

^* Three out of 16 administrative regions in Denmark—Copenhagen, Fyn, and Frederiksberg—had mammography screening in the 1990s. All three regions were excluded from the national and the historical national control groups.