Dealing adequately with technical uncertainties

Statistics, RAMS & Quality Management
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What does MTBF mean?
  What does MTBF mean
  How do MTBF calculations work
  MTBF calculations according to standards
  Cost of MTBF calculations

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.

Key statements

1. The MTBF of an item is mainly a measure of the complexity of that item, which boils down to this: The more components the item has, the worse its MTBF.
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 characterize 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 need attention during the system's lifetime, can be handled in two different ways:
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 established.

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