In What Ways are Designers’ and Operators’ Reasonable-World Assumptions Not Reasonable Assumptions?

Although the technical reliability of complex process plant generally continues to improve, such systems remain vulnerable to unmet assumptions on the part of designers, operators and maintenance staff. The purpose of this study was to characterize the nature of these assumptions, particularly those that were implied in people's activity rather than those that were explicitly invoked. Inferences about the assumptions were made from a set of accident reports. This characterization was then used as the basis of a tool that could help people examine how either their own or other peoples’ assumptions could imperil a system. Such a tool both structures the process of testing assumptions and helps synthesize the knowledge provided by historical failures. The tool was evaluated with a variety of organizations and used in a case study, where it contributed to the process of risk analysis in a variety of ways.