1. A method of diagnosing faults, comprising: detecting a current fault by a statistical process monitoring apparatus; determining one or more process variables that contributed to the current fault; calculating a relative contribution set based on a relative contribution to the current fault of each of the one or more process variables; comparing the calculated relative contribution set to stored relative contribution sets, wherein each of the stored relative contribution sets defines a fault signature, each stored relative contribution set including stored relative contributions of process variables having greatest statistical contributions to a specific fault; determining a fault signature that matches the fault, the stored relative contribution set of the determined fault signature including relative contribution ranges, wherein the relative contribution ranges are based on overlapping relative confidence ranges of at least two process variables, the fault signature matching the fault if the relative contributions in the calculated relative contribution set are within the relative contribution ranges; and identifying a fault class that defines a cause of the current fault.