# MTBF versus MTBCF

What is the difference between MTBF and MTBCF?

Usually, mean time between critical failure (MTBCF) is a term used when redundancy exists in a system. It is often used to differentiate system reliability from series mean time between failure (MTBF).  Series MTBF, or simply MTBF typically includes all failures without regard to any fault tolerance that may exist, whereas, the “C” in MTBCF indicates that only “critical” failures are counted, i.e., those that will cause the system to not meet specification requirements. That said, MTBF is sometimes used to really mean MTBCF.  This is often  organization and/or industry dependent – if unsure, ask!

# Reliability Theory

Most modern engineering disciplines are based on applied mathematics. An engineer or scientist observes a particular event and formulates a hypothesis (or conceptual model) which describes a relationship between the observed facts and the event being studied. In the physical sciences, conceptual models are, for the most part, mathematical in nature. Mathematical models represent an efficient, shorthand method of describing an event and the more significant factors which may cause, or affect, the occurrence of the event. Such models are useful to engineers since they provide the theoretical foundation for the development of an engineering discipline and a set of engineering design principles which can be applied to cause or prevent the occurrence of an event.