Reliability engineering is a sub-discipline of systems engineering that emphasizes the ability of equipment to function without failure. Reliability describes the ability of a system or component to function under stated conditions for a specified period of time. Reliability is closely related to availability, which is typically described as the ability of a component or system to function at a specified moment or interval of time.
The Reliability function is theoretically defined as the probability of success as at time t, which is denoted R(t). This probability is estimated from previous data sets or through reliability testing. Availability, Testability, maintainability and maintenance are often defined as a part of "reliability engineering" in reliability programs. Reliability can plays a key role in the cost-effectiveness of systems; for example, a consumer product in many cases will have a higher resale value, if it fails less often.
Reliability and quality are closely related. Normally quality focuses on the prevention of defects during the warranty phase whereas reliability looks at preventing failures during the useful lifetime of the product or system from commissioning, through operation, to decommissioning.
Reliability engineering deals with the prediction, prevention and management of high levels of "lifetime" engineering uncertainty and risks of failure. Although stochastic parameters define and affect reliability, reliability is not only achieved by mathematics and statistics. Reliability engineering can be achieved through process and reliability testing. "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." For example, it is easy to represent "probability of failure" as a symbol or value in an equation, but it is almost impossible to predict its true magnitude in practice, which is massively multivariate, so having the equation for reliability does not begin to equal having an accurate predictive measurement of reliability.
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. It can be said that a system must be reliably safe.
Reliability engineering focuses on costs of failure caused by system downtime, cost of spares, repair equipment, personnel, and cost of warranty claims.