Reliability engineering is engineering that emphasizes dependability in the lifecycle management of a product. Dependability, or reliability, describes the ability of a system or component to function under stated conditions for a specified period of time. Reliability engineering represents a sub-discipline within systems engineering. Reliability is theoretically defined as the probability of success (Reliability=1-Probability of Failure), as the frequency of failures, or in terms of availability, as a probability derived from reliability and maintainability. Maintainability and maintenance is often defined as a part of “reliability engineering” in Reliability Programs. Reliability plays a key role in the cost-effectiveness of systems. Reliability engineering deals with the estimation and management of high levels of “lifetime” engineering uncertainty and risks of failure. Although stochastic parameters define and affect reliability, according to some expert authors on Reliability Engineering, e.g. P. O’Conner, J. Moubray and A. Barnard, reliability is not (solely) achieved by mathematics and statistics. “Nearly all teaching and literature on the subject emphasize these aspects, and ignore the reality that the ranges of uncertainty involved largely invalidate quantitative methods for prediction and measurement” Reliability engineering relates closely to safety engineering and to system safety, in that they use common methods for their analysis and may require input from each other. Reliability engineering focuses on costs of failure caused by system downtime, cost of spares, repair equipment, personnel and cost of warranty claims. Safety engineering normally emphasizes not cost, but preserving life and nature, and therefore deals only with particular dangerous system-failure modes. High reliability (safety factor) levels also result from good engineering, from attention to detail and almost never from only re-active failure management (reliability accounting / statistics). A former United States Secretary of Defense, economist James R. Schlesinger, once stated: “Reliability is, after all, engineering in its most practical form.”