[time-nuts] MTBF (was Rubidium standard)

[Alan Melia]

Sorry Mike , unless, as someone else said, the figures are derived from
field failures over at least a good porton of the expected like the MTBF
tells you absolutely nothing!! The statistics used on the usual 1000hour
test will only tell you the probability of failure in the first 1000hours of
use!! It cannot tell you anything mathematically about the extrapolated
life....this has become another urban myth. If it works it is more by luck
that by mathematical probability.

Alan G3NYK


----- Original Message -----
From: "Mike S" <mikes at flatsurface.com>
To: "Discussion of precise time and frequency measurement"
<time-nuts at febo.com>
Sent: Wednesday, November 18, 2009 7:06 PM
Subject: Re: [time-nuts] MTBF (was Rubidium standard)


> 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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