Quality and reliability defined in different ways
Definition 1: Conformance to requirements—given by Crosby Definition 2: Freedom from deficiencies or fitness for use—given by Juran Definition 3: Degree to which a set of inherent characteristics of a product fulfils requirements—given by International Standards Organization
The probability that an item performs its ‘designed function’ for a ‘stated period of time,’ when operated under ‘stated conditions.’ It is a number between 0 and 1.
Let us try to understand statistics-based probabilistic definition of reliability more clearly. Say, a manufacturer sold a thousand refrigerators in a day all over the country. All started functioning on the same day. All were used as per the prescribed operating procedure in the prescribed environment for three years. Let us assume that 30 refrigerators in the batch failed in three years. It means that the probability of survival for three years is (100030)/1000=970/1000=0.97.
Hence 0.97 is the reliability for three-year time period for these refrigerators. The higher the reliability, lesser the probability of failure.