Assuming that the true proportion of thrombotic events is equal to 2.3% for both regimens (6-month and 12-month clopidogrel) 2000 patients for each treatment group are necessary to demonstrate a non-inferiority of the 6-months regimen if the proportion of thrombotic events will be no more than 3.5% with a power of 0.80 and a significance level of 0.05 (one-tail).