What is "reliability"

Reliability \Re*li`a*bil"i*ty\ (r?-l?`?-b?l"?-t?), n. The state or quality of being reliable; reliableness.

1816, from reliable + -ity.

n. 1 The quality of being reliable, dependable(,) or trustworthy. 2 The quality of a measurement indicating the degree to which the measure is consistent, that is, repeated measurements would give the same result (See validity). 3 In engineering measurable time of work before failure

n. the trait of being dependable or reliable [syn: dependability, dependableness, reliableness] [ant: undependability, undependability, undependability, undependability]

Reliability may refer to:

• Human reliability
• Reliability engineering, the ability of a system or component to perform its required functions under stated conditions for a specified time.
• Reliability (statistics), the overall consistency of a measure
• High reliability is informally reported in " nines"
• Reliabilism in philosophy and epistemology
• Data reliability, a property of some disk arrays in computer storage
• Reliability theory, as a theoretical concept, to explain biological aging and species longevity
• Reliability (computer networking), a category used to describe protocols
• Reliability (semiconductor), outline of semiconductor device reliability drivers
• Reliability (research methods), the "consistency" or "repeatability" of research measures.

Reliability in statistics and psychometrics is the overall consistency of a measure. A measure is said to have a high reliability if it produces similar results under consistent conditions. "It is the characteristic of a set of test scores that relates to the amount of random error from the measurement process that might be embedded in the scores. Scores that are highly reliable are accurate, reproducible, and consistent from one testing occasion to another. That is, if the testing process were repeated with a group of test takers, essentially the same results would be obtained. Various kinds of reliability coefficients, with values ranging between 0.00 (much error) and 1.00 (no error), are usually used to indicate the amount of error in the scores." For example, measurements of people's height and weight are often extremely reliable.

In computer networking, a reliable protocol provides reliability properties with respect to the delivery of data to the intended recipient(s), as opposed to an unreliable protocol, which does not provide notifications to the sender as to the delivery of transmitted data. The term "reliable" is a synonym for assured, which is the term used by the ITU and ATM Forum in the context of the ATM Service-Specific Coordination Function, for example for transparent assured delivery with AAL5.

Reliable protocols typically incur more overhead than unreliable protocols, and as a result, function more slowly and with less scalability. This often is not an issue for unicast protocols, but it may become a problem for reliable multicast protocols.

TCP, the main protocol used on the Internet, is a reliable unicast protocol. UDP, often used in computer games or in other situations where speed is an issue and the loss of a little data is not as important because of the transitory nature of the data, is an unreliable protocol.

Often, a reliable unicast protocol is also connection-oriented. For example, TCP is connection-oriented, with the virtual-circuit ID consisting of source and destination IP addresses and port numbers. Some unreliable protocols are connection-oriented as well. These include ATM and frame relay. There are also reliable connectionless protocols, such as AX.25 when it passes data in I-frames. But this combination occurs rarely: reliable-connectionless is uncommon in commercial and academic networks.

Reliability in research methods concerns the quality of measurement. Reliability is the extent to which a research method produces the same results each time it is applied to the same study. Reliability refers to the "repeatability" or "consistency" of research measures. Reliability refers to the extent to which a research method produces the same results each time same method is applied.

Reliability of semiconductor devices can be summarized as follows:

1. Semiconductor devices are very sensitive to impurities and particles. Therefore, to manufacture these devices it is necessary to manage many processes while accurately controlling the level of impurities and particles. The finished product quality depends upon the many layered relationship of each interacting substance in the semiconductor, including metallization, chip material ( list of semiconductor materials) and package.
2. The problems of micro-processes, and thin films and must be fully understood as they apply to metallization and wire bonding. It is also necessary to analyze surface phenomena from the aspect of thin films.
3. Due to the rapid advances in technology, many new devices are developed using new materials and processes, and design calendar time is limited due to non-recurring engineering constraints, plus time to market concerns. Consequently, it is not possible to base new designs on the reliability of existing devices.
4. To achieve economy of scale, semiconductor products are manufactured in high volume. Furthermore, repair of finished semiconductor products is impractical. Therefore, incorporation of reliability at the design stage and reduction of variation in the production stage have become essential.
5. Reliability of semiconductor devices may depend on assembly, use, and environmental conditions. Stress factors affecting device reliability include gas, dust, contamination, voltage, current density, temperature, humidity, mechanical stress, vibration, shock, radiation, pressure, and intensity of magnetic and electrical fields.

Design factors affecting semiconductor reliability include: voltage derating, power derating, current derating, metastability, logic timing margins ( logic simulation), timing analysis, temperature derating, and process control.