1. A method for updating a fault tree used in diagnosis of a condition of a machine, the fault tree comprising a sequence of test modules, each of the test modules being assigned a confidence score, the method comprising the steps of: obtaining service data from a plurality of service occasions for like machines; revising the confidence score for at least one test module in the fault tree based on the service data; and revising the sequence of the test modules in the fault tree based on the revised confidence score(s).