The synergistic twins: fault trees and success trees

Failures and cost-overruns of many projects could be prevented by a clear and timely definition of what should be achieved and what should be avoided. A powerful tool for the development of a clear definition is a combination of the success tree and the fault tree. The paper describes the application of that approach. It shows how the graphic format helps to amalgamate deductive analyses with inductive experience, and notes the difference in thinking when developing a success tree as opposed to a fault tree. It demonstrates how a comparison of the two forms of the tree facilitates identification of forgotten factors, and describes some of the idiosyncrasies in the conversion of the tree to its dual form. The paper also mentions the hybrid form of a success tree with failure data, which proved useful on several occasions, but requires special precautions to prevent errors caused by a mix-up with regular success trees or fault trees. The combination of success trees and fault trees for a proper definition of goals and possible problems has been successfully applied in the aviation and telecommunication fields, but very little has been written about it. This approach would be particularly useful in robotics and other hi-tech fields, where designers tend to rely on iterations to get it right. The paper should encourage reliability practitioners to promote the use of reliability techniques as design tools.