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  2. Mean Time Between Failure (MTBF)

Mean Time Between Failure (MTBF)

ALCL 085


A basic measure of reliability for repairable items. The mean number of life units during which all parts of the item perform within their specified limits during a particular measurement interval under stated conditions.

General Information

In addition to the Glossary definition above, DoD Handbook (DoD-HDBK) 791Military Handbook on Maintainability Design Techniques, describes MTBF as “the total functioning life of a population of an item divided by the total number of failures within the population during the measurement interval. ” The explanation holds for time, rounds, miles, events, or other measures of life units. Additionally, MTBF is a basic technical measure of Reliability (R) recommended for use in the Research and Development (R&D) contractual specification environment, where “time” and “failure” must be carefully defined for contractual compliance purposes.

Further, in accordance with Quantitative Measurements For Logistics by Philip T. Frohne, MTBF is a measure of equipment R measured in equipment operating hours and describes failure occurrences for both repairable and non-repairable items.

According to the Integrated Logistics Support Handbook by James V. Jones, also states that MTBF is considered to be another expression of R. This reference also notes that the calculation of MTBF is accomplished using the same data as calculating a failure rate. Since both the MTBF and the failure rate are calculated using identical data, they are therefore, the same basic statistics presented in two different ways. The failure rate predicts the average number of hours that an item, assembly, or piece part will operate before it fails, while the MTBF predicts the average elapsed time between failures.

Jones posits that the MTBF of an item is often used in calculating spares requirements, Life Cycle Cost (LCC) and other failure related data. He further asserts that practice has shown that when talking about the reliability of an item it is easier to use an MTBF rather than a failure rate since we tend to be more comfortable with whole numbers rather than with decimals.