What is Reliability?


Military standard definition of reliability

"The probability that an item will perform a required function without failure under stated conditions for a stated period of time."

Note:

Reliability objectives

Design objectives that refers to an item's reliability.

Examples:

Reliability Objectives in Ancient Egypt

"As concerns the gold ring set with an Emerald, we guarantee that for 20 years the emerald will not fall out of the gold ring. If the emerald should fall out of the gold ring before the end of 20 years, we shall pay unto Bel-Nadin-Shumu and indemnity of 10 mana of silver."

Record found on a clay tablet in Egypt - 429. BC

Notice that except for probability, all of the element in the definition of reliability are found.

Major Division of Reliability Methods

System Reliability
(Reliability Prediction)

Probability methods are used to determine system reliability using knowledge of system architecture and part reliability.

Part Reliability
(Reliability Estimation)

Statistical methods are used to determine part reliability using part failure data (reliability data). A part is an item that is assumed to be indivisible for the purpose of determining its reliability

Repairable System Reliability

Use of reliability figures of merit that require some knowledge of system repair activity.

Non-Repairable System Reliability

Use of reliability figures of merit that do not require knowledge of repair activity (if any)