We have chosen to base our treatment recommendations on absolute rates of stroke derived from clinical trials, recognizing that these data have important limitations. Less than 10% of patients screened were enrolled in these historical trials, there was limited racial and ethnic diversity, and there is some evi- dence suggesting that stroke rates may now be lower than at the time these RCTs were conducted 2 decades ago possibly because of improved treatment of cardiovascular risk factors, such as hypertension.51,52 However, despite these limitations, these clinical trial-based data regarding stroke events were systemati- cally and prospectively collected, and they remain the best avail- able source of stroke rates strati ed by CHADS2 score. 1.5 Deriving Baseline Risk of Death in Patients With AF We used data from an observational health plan database study of 11,526 patients with nonvalvular AF (the Anticoagula- tion and Risk Factors in Atrial Fibrillation [ATRIA] Study) to obtain an estimate of the risk of all-cause mortality in patients with AF not treated with warfarin (53 deaths per 1,000 patient- years). Untreated patients in this cohort had a mean CHADS2 www.chestpubs.org CHEST / 141 / 2 / FEBRUARY, 2012 SUPPLEMENT e539S Table [Sections Should VKAs Rather Than No Therapy Be Used in Patients With AF? a Quality Assessment Summary of Findings Risk of Bias Inconsistency Indirectness Imprecision Publication Bias Study Event Rates (%) Estimation of Absolute Effects y Time Frame No. 