| Term | Definition |
| Availability | A measure of the degree to which an item is in an operable state at any time. |
| Benchmarking | Rating a company's practices, designs, processes against the world's best practices for purposes of seeking improvement. |
| Built-in Test (BIT) | An integral capability designed into a product which provides an automated test capability to detect or isolate failures. |
| Consumer's Risk (β) | accepting an item which would be proven bad if the test was conducted for an infinite time (or population). |
| Control Charts | Statistical charts derived from measuring factory processes. Used to spot process "drift" and inherent process variations which designers must account for in the basic design to achieve a "robust design." |
| Derating | Using an item in a way that applied stresses are below rated values. |
| Environmental Stress Screening (ESS) | A series of tests conducted under environmental stress often greater than experienced in normal operation to disclose weak parts and workmanship defects to be corrected. |
| Failure Rate (λ) | The total number of failures within an item population, divided by the total time expended by that population, during a particular measurement interval under stated conditions. |
| Fault Detection | A process which discovers the existence of faults. Can be accomplished manually or automatically, depending on product requirements. |
| Fault Isolation | The process of determining the location of a fault to the extent necessary to effect repair. Can be accomplished manually or automatically, depending on product requirements. |
| Hazard Rate | Instantaneous failure rate. At any point in the life of an item, the incremental change in the number of failures per associated incremental change in time. |
| Maintainability | A measure of the ability of an item to be retained in, or restored to, a specified condition when maintenance is performed using prescribed procedures and technician skill levels. |
| Mean-Downtime | The average time a system is unavailable for use due to a failure. Time includes the actual repair time plus all delay time associated with a repairman arriving with the appropriate replacement parts: |
| Mean-Time-Between-Failure (MTBF) | A basic measure of reliability for repairable items. The average time during which all parts of the item perform within their specified limits, during a particular measurement period under stated conditions. |
| Mean-Time-Between-Maintenance (MTBM) | A basic measure of reliability for repairable fielded systems. The average time between all system maintenance actions. Maintenance actions may be for repair or preventive purposes. |
| Mean-Time-Between-Repair (MTBR)
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A basic measure of reliability for repairable fielded systems. The average time between all system maintenance actions requiring removal and replacement of a box or subsystem. |
| Mean-Time-Between-Critical-Failure (MTBCF) | A measure of system reliability which includes the effects of any fault tolerance which may exist. The average time between failures which cause a loss of a system function defined as "critical" by the customer. |
| Mean-Time-To-Failure (MTTF) | A basic measure of reliability for nonrepairable systems. Average failure free operating time, during a particular measurement period under stated conditions. |
| Mean-Time-To-Repair (MTTR) | A basic measure of maintainability. The sum of corrective maintenance times divided by the total number of failures within an item. The average time takes to fully repair a failed system. Typically includes fault isolation, remove and replacement of failed item(s) and checkout. (Also called mean corrective maintenance time, Mct). |
| Mean-Time-To-Restore-System (MTTRS) | The average time it takes to restore a system from a failed state to an operable state. Time includes both delay time associated with obtaining parts required and actual repair time. |
| MMAX | The maximum repair time associated with some percentage of all possible system repair actions. (e.g., 95% of all repair actions must be accomplished in less than one hour). |
| Preventive Maintenance (PM) | Time associated with the performance of all required preventive maintenance, both scheduled and unscheduled. Usually expressed in terms of hours per year. |
| Producer's Risk (α) | Used in conjunction with statistical testing. The probability of a customer rejecting an item which would be proven good if the test was conducted for an infinite time (or population). |
| Quality Funct Deployment | A system that focuses on exactly what the customer wants. Activities which don't contribute to customer goals are considered wasteful and are eliminated. |
| Redundancy | The existence of one or more means (not necessarily identical) for accomplishing a given function. Active redundancy has all items operating simultaneously, while standby redundancy has alternate means activated upon failure. |
| Reliability | The probability that an item will perform its intended function for a specified interval under stated conditions. |
| Reliability Growth | The improvement in reliability caused by the successful correction of deficiencies in an item design or manufacture. |
| Robust Design | A design approach that accounts for limitations in production capabilities, such as accounting for production machinery tolerance limitations. |
| Testability | A design characteristic which allows the status of the unit to be confidently determined in a timely manner. |
Source:
SRC Publication, CPE, Reliability Toolkit: Commercial Practices Edition.
For More Information:
Prentice-Hall Report, 1961, Bazovsky, I., "Reliability Theory and Practice."
Connor, P., "Practical Reliability Engineering."
Springer-Verlag Report, 1994, Birolini, A., "Quality and Reliability of Technical Systems."
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