Introduction

The world we live in is imperfect and we increasingly have to live with failure of many of our traditional as well as advanced mechanical, electrical, structural, as well as economic and social systems. The reason often is not insufficiency of concept, design, or operational standards used in the system, but the fact that little or no consideration has been given to the desired reliability, availability, and maintainability of the system in its conception, design, and operation. It is not only that we often expect too much, but that we are ignorant of the actual operating environment and required performance imposed on a system. Little or no consideration is usually given to the basic fact that “nothing is perfect”, and the system’s design as well as operating conditions are subject to many deviations and uncertainties. It is therefore necessary to assign inherent or potential imperfections to systems so as to achieve desired performance.