Reliability analysis of systems based on software and human resources

Safety critical systems require an assessment activity to verify that they are able to perform their functions in specified use environments. This activity would benefit from evaluation methodologies that consider these systems as a whole and not as the simple sum of their parts. Indeed, analysis of accidents involving such systems has shown that they are rarely due to the simple failure of one of their components. Accidents are the outcome of a composite causal scenario where human, software and hardware failures combine in a complex pattern. On the contrary, dependability analysis and evaluation of safety critical systems are based on techniques and methodologies that concern human and computer separately, and whose results can hardly be integrated. The analogies between the processes of: (1) software reliability growth due to testing and the related fault removal; (2) improvement of man machine interface due to preliminary operative feedback; (3) improvement of the operator performances due to his learning activity; suggest an effort for a common evaluation approach. Only the first one of these processes is currently modelled by using mathematical methods. The paper considers extending these methods to study the reliability growth process of other system components, i.e. the operator and the man machine interface. To study the feasibility of the approach, the paper analyses the results of an experiment in which the reliability of a system is evaluated using trend analysis. The evaluation concerns the graphic man machine interface and the operators, and could easily be extended to the software control system