Validity is the extent to which a test accurately measures what it is supposed to measure. Reliability engineering is often used in conjunction with quality assurance and safety engineering to ensure that products are safe and reliable. Reliability engineers use tools and techniques such as statistical analysis and probability theory to identify potential sources of failure and to estimate the reliability of systems. Reliability engineering is a sub-discipline of systems engineering that focuses on maximizing the reliability of a system.In other applications, such as consumer electronics, reliability may be less important than cost or other factors. For example, in the design of safety-critical systems such as aircraft or medical devices, reliability is given paramount importance.In many cases, reliability is more important than other performance metrics such as cost or speed.

Reliability is an important consideration in the design of any system, and it is often included as a performance metric in engineering specifications. Reliability is often quantified by measuring the average time between failures (MTBF) or by measuring the likelihood of failure on demand (Ft).