The reliability of a component or system can be represented in a statistical sense by the probability of a component or system performing satisfactorily at a particular time under a specified set of operating conditions. The definition of what constitutes ‘satisfactory’ may depend upon the nature of the system. Some devices, such as switches and valves, may have only an ‘operate’ or ‘non-operate’ mode. Other devices may be judged satisfactory or not depending on the required output level of some performance variable such as power or thrust. The present introductory discussion will consider the first two of the following four aspects of reliability;

1. The change in reliability of a component or system with age 2. The reliability of a system as influenced by the arrangement of components 3. The precision of estimates of reliability and other associated reliability parameters 4. The ability of a product to perform within specified limits under the influence of some

external stress or environment

The object of the present discussion is to introduce some basic concepts and complete treatments can be found in the various references cited.