::In response to the thread from David below: If 1,000,000 devices are
tested for one hour and 100% of the population survives the test, the MTBF =
1,000,000 hours by definition. That is the statistical mean, as a minimum,
since no devices failed. If they *all* fail one minute later, the MTBF
based on the original test is still 1,000,000 hours because the failures can
be ignored: The test was concluded, and MTBF already calculated. This is
exactly and specifically allowed by both the Bellcore-3 standard and
MIL-HDBK-217 standard for calculating MTBF, and those are the industry
norms, at least here in the U.S.
To be fair, normally during MTBF testing a quantity of devices is tested
until some failures actually do occur, and do not stop after one hour. This
yields are far more accurate picture, of course. However, it would be
perfectly legitimate to establish the time standard in advance (say, "one
hour") and test a population, and calculate MTBF based on that.
-WB2WIK/6
+++++++++++++++++++++++++++++++++++++++++++++=
I am however puzzled by the comments of WB2WIK/6 stevek@jmr.com when he
states this:
> MTBF is a funny statistical tool: Per even
> MIL-HDBK-217 and Bellcore standard definitions,
> MTBF may be determined in any way that
> arithmetically accomplishes the calculaton of a mean.
> Including this one: 1 million hours MTBF
> may be described as having 1 million
> devices in operation for one hour
> each, with failure rate of one
> device or less.
> If 100% of the population all failed
> after 61 minutes, this lot would still
> have an MTBF of 1 million hours by that definition.
I can't for the life of me understand how if 100% of devices in a sample
of 1,000,000 all failed within 61 mintes can you draw the conclusion
that the MTBF is 1,000,000 hours. I would have thought that is
statistically valid data to prove the MTBF is <= 61 minutes, rather than
1,000,000 hours.
Even if 100% work after 60 minutes, the confidence that you could put on
any one device lasting 1,000,000 hours is negligsable.
I have not however read the mil spec - I only looked at the Seagate
(might be another disk manufacturer) and determined the MTBF, which
might suggest disks last on average for 50-150 years is an incorrect
interpretation.
You know the old proverb - there are lied, damm lies and then there are
statistics!
--
David Kirkby,
G8WRB
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