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:

A functional definition of failure is needed. For example, a failure means different things to the user and to the repair person.
The system's operating conditions must be specified (e.g. car or home environment).
A period of time is needed.

Reliability objectives

Design objectives that refers to an item's reliability.

Examples:

For telephone switching equipment: Less than two hours of downtime in 40 years.
For submarine light guide system (TAT - 8): Less than three failures requiring ship repair in 25 years.
For an AT&T PC (around 1987): A Mean Time Between Failures (MTBF) of at least 2500 hours.

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)