adequately with technical uncertainties
& Quality Management
What does MTBF mean?
MTBF means Mean Time between
Failures, in other words, the average time between two consecutive
failures (of whatever item).
Although MTBF seems to be a handy and easy metric at first glance, some
interpretation is required in order to get the picture.
1. The MTBF of an item is mainly a measure of the complexity of that
which boils down to this: The more components the item has, the worse
In the real world, system architecture, failure tolerance,
maintenance, etc., would have significant influence on the
avarage time between failures, but the MTBF model simply ignores them.
Therefore, MTBF is very often only a humble means in order to
the "real" reliability of an item.
2. With a few rare exceptions, MTBF has no
relationship with lifetime, and these two (MTBF and Lifetime) cannot
even be transformed into
each other. In fact, MTBF means the average time between two
consecutive failures during the
useful product life. See the so called bath tub curve for reference.
3. MTBF values of single components (resistors, transistors, screws,
bolts, etc.) are usually by several orders of magnitudes
higher than the lifetime of the system in which they are built in.
4. Components like batteries and other special components that would
attention during the system's lifetime, can be handled in two different
a) Corrective maintenance: Critical components are replaced after
failure. In this philosophy, the system MTBF will be dominated by the
battery lifetime, resulting in a quite low system MTBF.
b) Preventive maintenance: Critical components are replaced before
they are likely to fail. In this case, the critical component lifetimes
probably don't affect the system's MTBF at all, resulting in a quite
high system MTBF.
Note: It is common practice to exclude critical components from system
MTBF provided that appropriate preventive
maintenance has been