Introduction
Observational studies have been conducted since the 1980s to evaluate the effectiveness of breast cancer screening programs. These studies compare the risk of breast cancer death between women attending and not attending screening mammography. Case-control studies retrospectively assess the attendance at screening mammography of case women who died from breast cancer and of control women who were still alive when case women died from breast cancer. Cohort studies prospectively compare the risk of breast cancer deaths of women attending screening to the risk of women who do not attend screening. However, these studies are prone to self-selection bias (or “healthy user bias”). Women not attending screening are generally less educated and less health-aware, are more deprived, and have more comorbidities…