Probability as a measure of reliability

Let us suppose that reliability is defined simply as the probability that an item can perform a required function under the stated conditions for a specified period of time. Reliability can then be expressed by some number lying between zero and unity, where zero represents certain failure and unity certain success in the accepted manner. In this concept an item either succeeds or fails — there is no halfway house. (This matter is further discussed in section 7.1.) We will denote reliability, as the probability of success, by the symbol R, and the probability of failure by the symbol F.