Summary of meta-analysis results by design and age screened^a.

a. Each cell represents the RR and 9% CI for a separate meta- computed using the metan random effects model in Stata. The historical comparisons included some studies with prevalence-based breast cancer. For historical designs, both incidence and prevalence-based breast cancer are reported but in separate rows. Incidence-based breast cancer excludes women diagnosed with breast cancer before the screening program was initiated. Prevalence-based breast cancer does not.