The method includes obtaining system model data representing a set of failures in a system including a plurality of components, a set of symptoms and relationships between at least some of the failures and symptoms. The system model data is used to create a Bayesian Network. Failure cases data is also obtained, where each failure case describes the presence/absence of at least one of the symptoms and the presence/absence of at least one of the failures. A learning operation on the Bayesian Network using the failure cases data is then performed and the contribution made by at least some of the failure cases to updating the parameters of the Bayesian Network during the learning operation is assessed. Information representing the assessed contribution of the at least some failure cases is displayed.