Fri Oct 9 21:52:43 UTC 2009
equivalents were never more than a method for comparing the design
revisions. The relationship to reality is quite bogus unless your tests are
an exact replication of the world the item will experience. Cost and
available time, not to mention the fact that each unit will have a different
life experience ( just like people), makes testing in the actual environment
difficult, and renders any extrapolation of test results to reality an
exercise in fantasy. Which explains why the marketing and upper management
types love it.
Most testing that produces MTBF figures is also based on hitting the
extremes of the environment such as temperature, rate of change of
temperature, and humidity. These usually are only loosely related to
reality, but are based, hopefully, on some real world data. It is the
correlation factor that is suspect.
My contention is that MTBF is only valid when based on data from the units
operating in the field, and is essentially after-the-fact. By that time,
what's done is done. If you have real world experience with a statistically
valid number of units, then you can determine MTBF with some confidence.
Otherwise, it is just a SWAG. But it can be a useful SWAG for comparison
Frankly, I would distrust any outfit that puts it into their advertising
without some sort of disclaimer regarding how it was determined, and the
Tom Holmes, N8ZM
Tipp City, OH
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Alan Melia
Sent: Wednesday, November 18, 2009 9:27 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Rubidium standard
The big problem with MTBF is that it doesnt really mean ANYTHING if you
invoke the proper statistical properties of the calculation! It is a process
dreamed up out of thin air by Military and other users who felt they needed
an index of quality and at least some "life testing" on the product they
were buying without elevating the price too much, as proper life tests
would. Mathematically it is highly suspect, but that depends on how the
figures are used and most involved....management level :-)) dont understand
Statistics....... so they are invariably mis-used !
----- 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 12:54 PM
Subject: Re: [time-nuts] Rubidium standard
> At 05:22 AM 11/18/2009, Steve Rooke wrote...
> >The point I should have made is that most quoted MTBF figures have a
> >reasonable bearing on the lifetime of the item,
> But your point would then be almost perfectly incorrect. MTBFs are not
> meant to, nor do they, predict product lifetimes. They are
> measures/predictions of product reliability.
> "What does MTBF have to do with lifetime? Nothing at all!" -
> "MTBF represents the statistical approximation of how long a number of
> units should operate before a failure can be expected. It is expressed
> in hours and does not represent how long the unit will last." - Learn
> (or review) the difference between MTBF and lifetime, Control
> Engineering, 9/24/2008;
> I don't grant Wikipedia strong authority, but it is useful, and has
> this to say: "MTBF is commonly confused with a component's useful life,
> even though the two concepts are not related in any way. For example a
> battery may have a useful life of four hours, and an MTBF of 100,000
> hours. These figures indicate that in a population of 100,000
> batteries, there will be approximately one battery failure every hour
> during a single battery's four-hour life span."
> There's much more out there, if you make the effort.
> >I felt that an example based on humans was not really applicable to
> >the real world of electronic items but that is my own opinion and I'm
> >happy if you disagree with me.
> MTBFs are not exclusive to electronics. Statistics, math and MTBFs are
> objective matters, so your opinion really doesn't make any difference.
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