In contrast , the 30-day risk of stroke or death was significantly higher in patients treated by operators with low ( mean 3.2 procedures/y ; risk 10.1 % ; adjusted risk ratio = 2.30 [ 1.36-3 .87 ] ) and intermediate annual in-trial volumes ( 3.2-5 .6 procedures/y ; 8.4 % ; adjusted risk ratio = 1.93 [ 1.14-3 .27 ] ) compared with patients treated by high annual in-trial volume operators ( > 5.6 procedures/y ; 5.1 % ) .