System Reliability and Risk Assessment: A Quantitative Extension of IDEF Methodologies

1. Introduction Evaluating system reliability requires modeling the interaction of resources, information, and material within the system. Such a model must consider quantitative data describing the reliability of each element of the system, as well as logical data describing the relationship between individual components. For example, a manufacturing system may assemble products X, Y, and Z, on machines M1, M2, and M3, respectively, and package the products on a fourth machine, M4. Therefore, three different relationships exist between the components of the system, one for each product. If the reliability of each machine differs, the system reliability will differ depending on the product. Similarly, the reliability of the system as a whole will be affected by the production levels of products X, Y, and Z. Given the example above, with only three products and four machines, it becomes apparent that determining system reliability requires a significant amount of data and a structured modeling methodology. Furthermore, to obtain an accurate assessment of system reliability, it is necessary to include additional data describing information and material components of the system; such as inspection procedures, assembly specifications, parts, and subassemblies. System reliability describes the likelihood of success or failure in the operation of a system. Risk assessment is a technique for identifying scenarios that lead to problems in a system, determining the likelihood of each scenario, and the evaluating the consequence of each scenario, i.e., the problem. Although quantitative approaches to risk assessment employ the principles of system reliability, the identification, quantification, and evaluation of risk is a more comprehensive modeling activity. Therefore, risk assessment projects are well suited for the tools developed for process modeling and analysis. In 1978, the United States Air Force selected SADT (Structured Analysis and Design Technique) as the language to support the Integrated Computer Aided Manufacturing (ICAM) program. SADT activity modeling was adopted by the ICAM program and revised by Sofrech, Inc. to develop the ICAM Definition Methodology (IDEF0). Ross (1985) states that "thousands of people from hundreds of organizations working on more than one hundred major projects" proceeded to use the methodology for system definition and design, as well as project management. IDEF0 introduced manufacturing to techniques that were developed for computer system and software engineering applications. Additional IDEF techniques were developed