Requirements analysis using forward and backward search

The requirements analysis of critical software components often involves a search for hazardous states and failure modes. This paper describes the integration of a forward search for consequences of reaching these forbidden modes' with a backward search for contributing causes. Results are reported from two projects in which the integrated search method was used to analyze the requirements of critical spacecraft software. The search process was found to be successful in identifying some ambiguous, inconsistent, and missing requirements. More importantly, it identified four significant, unresolved requirements issues involving complex system interfaces and unanticipated dependencies. The results suggest that recent efforts by researchers to integrate forward and backward search have merit.