[time-nuts] Rubidium standard

[Alan Melia]

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 !

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 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!" -
> http://www.ece.cmu.edu/~ganger/ece546.spring02/readings/mtbf.description
>
> "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;
>
http://www.controleng.com/article/312365-Learn_or_review_the_difference_betw
een_MTBF_and_lifetime.php
>
> 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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