Sample size: By using estimates of event rates in the described patient population (thromboembolism event rate of 1.4% per year, mainly on VKA; unpublished data) and a relative risk reduction of 0.81 for stroke or systemic embolism on NOACs compared to VKA [Ruff et al, Lancet 2013], a sample size of 300 patients followed for two years is necessary to demonstrate the safety of NOACs for thromboembolic prevention with a safety margin of 2.7% thromboembolic events per year.