Negative controls to detect uncontrolled confounding in observational studies of mammographic screening comparing participants and non-participants

Background When comparing mammography-screening participants and non-participants, estimates of reduction in breast-cancer mortality may be biased by poor baseline comparability. We used negative controls to detect uncontrolled confounding. Methods We designed a closed cohort of Danish women invited to a mammography-screening programme at age 50–52 years in Copenhagen or Funen from 1991 through 2001. We included women with a normal screening...