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The expected rate of invasive breast cancer over at two year period for the age group 50-69 years in Norway is 400 in the absence of mammography screening. The observed cumulative rate of invasive breast cancer in a 2 year period after a mammography screening in Norway is 190.

The absolute number of interval cancers is very similar in different national screening programmes (for example in the Dutch screening programme the rate of interval cancer was 180 cases per 100 000 per 2 year). For un-known reasons the rate of interval cancers in the Swedish Two-County Study was only half of this number.

After a group of women have been screened their rate of breast cancer is typically reduced by 70% the first year, 30% the second year. The rate is reduced by only 15% the third year and tend to the background level after four years. Therefore, time between screening rounds is 2 or 3 years.