1. A system for determining cardiovascular information for a patient, the system comprising: at least one computer system configured to: receive patient-specific data regarding a geometry of at least the patient's vasculature; create an anatomic model representing at least a portion of the patient's vasculature based on the patient-specific data; create a computational model of a blood flow characteristic based on the anatomic model; identify one or more of an uncertain parameter, an uncertain clinical variable, and an uncertain geometry; modify a probability model based on one or more of the identified uncertain parameter, uncertain clinical variable, or uncertain geometry; determine a blood flow characteristic within the patient's vasculature based on the anatomic model and the computational model of the blood flow characteristic of the patient's vasculature; and calculate, based on the probability model and the determined blood flow characteristic, a sensitivity of the determined blood flow characteristic to one or more of the identified uncertain parameter, uncertain clinical variable, or uncertain geometry.