1. A method for determining relative likelihood of a failure mode, the method comprising: receiving evidence observations of a monitored system from monitors connected in a many-to-many relationship to the failure modes; generating a fault condition including states of all failure modes that are connected to the monitors, the fault condition being generated for a reference model of the monitored system based on the received evidence observations; and computing a relative probability of failure for each failure mode based on a false alarm probability, a detection probability, and a ratio of prior probabilities of a candidate hypothesis to a null hypothesis of no active failure modes.