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

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.