MTBF ranges at least from 5.1 to 92.6 CPU hours. Because it is based on the occurrence of observable problems or failures in the product, it is among the easiest of the quality factors to measure. It is also an easy concept for a user to relate to - the user expects the product to be error free, thus, highly reliable. Maintainability and availability are two other quality factors closely related to reliability. The calculation of each of these is again, based on tracking failures.
Reliability is one of the best known and oldest metrics.
Software managers typically tracked software bugs even before software reliability concepts were explicitly formalized. Additionally, much work since the seventies has been directed toward conceptual clarification and modeling of software reliability. This has produced a consensus view of the software failure process, which is reflected in the definitions in Table 2.2-1. Basically, failures, observed while operating software, are caused when the software enters a state in which a fault or defect in the product changes the resulting output. Since inputs, which effect state changes, and the location of faults are not completely known a priori, software reliability is modeled as a stochastic process.