One embodiment of the invention provides apparatus including a data structure representing a fault tree for a system. The data structure comprises a plurality of events linked by propagations. Each event is classified as one of at least three possible event types. A first type of event is a problem event, which represents an underlying cause of misbehavior in the system. A second type of event is an error event, which represents an error in the system comprising an incorrect signal or datum. A third type of event is a report event, representing the formal detection by the system of an error. Each propagation in the fault tree denotes a cause and effect linkage from one event to another event. There are no propagations within the fault tree to a problem event.