Catalogue of potential threats to internal and external validity of breast cancer screening quasi-experimental studies stratified by specific designa.

a. List and definitions of threats of validity from Grembowski D. The Practice of Health Program Evaluation. Sage Publications: Thousand Oaks, CA; 2001.b. Study did not report the source of death data but it was assumed that they came from national health system and registries from the UK.c. Other forms of death verification: regional radiology departments, carcinoma working groups and panels of physicians.