4.5 Diagrams for diagnosis
As the detail of the connectivity revealed through a diagram increases, many diagrams can be used for diagnosis by comparing a diagram of what should be happening with what is happening. This approach has been developed in detail by Bignell and Fortune (1984) to analyse systems failures. They argue that all satisfactory systems have functioning decision-making, operational and performance monitoring systems and that many failures can be explained by a failure in one of these aspects, even when the other elements of the system were working satisfactorily. Other failures can be explained by weaknesses in connectivity between the elements of a system leading to ‘systemic failures’ – that is, failures in which individual elements of the system functioned satisfactorily in isolation, but the ways in which they were connected or interacted together led to a failure of the system as a whole.