[time-nuts] MTBF (was Rubidium standard)

Mike S mikes at flatsurface.com
Wed Nov 18 19:06:55 UTC 2009


At 01:14 PM 11/18/2009, Michael Sokolov wrote...
>As I have learned in school from a department head, mean time between
>failures (MTBF) means anything only if you are being mean.  If you are
>not being mean, it means nothing.

There is at least one practical use for MTBF, at least the real-world 
statistical form.

If one has a reasonably large population of devices to maintain, the 
number of spare devices needed to stay operational is a function of the 
MTBF.

If you've got 10,000 cell sites, each with an Rb timebase, MTBF figures 
can provide a pretty accurate estimate of how often one will fail and 
need to be replaced, at least during the normal lifetime (between the 
infant mortality and wearout stages). Coupled with MTTR statistics, one 
can figure out how many spares should be stocked.

Manufacturers invest a lot of effort into using such statistical 
measures to determine how many spare parts should be kept at various 
points in the supply chain. Unnecessary parts on shelves = wasted 
capital. Manufacturers also use MTBF statistics to price service 
contracts on equipment.

This only works when one is dealing with a large population of devices, 
so MTBF is meaningless at the individual unit level.




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