Formalising engineering judgement on software dependability via belief networks

We present the use of Bayesian belief networks to formalise reasoning about software dependability, so as to make assessments easier to build and to check. Bayesian belief networks include a graphical representation of the structure of a complex argument, and a sound calculus for representing probabilistic information and updating it with new observations. We illustrate the method and show its feasibility via a simple example, developed via a commercial computer tool, representing a form of argument which is often used in claims for high dependability. This example is not meant to be "typical", since a sound and complete argument can only be built using the knowledge available in the specific case of interest. Although the modelled scenario is rather simple, but it demonstrates the advantages of using belief networks for sounder assessment of reliability and safety.