Results of Multilevel Analysis: Cumulative volumes within the study period and SSIs were identified by the CART model.

OR: odds ratio; C.I.: confidence interval; LCL: lower confidence limit; UCL: upper confidence limit All models were adjusted by hospital ownership and geographic location, surgeon’s gender and age, patient’s age, gender, length of ICU stay, number of vessels obstructed and underlying diseases Results of Multilevel Analysis: Cumulative volumes within the study period and SSIs were identified by the CART model.