[Test-Equipment] Corrected link

[wolfbob]

I found two or three gross approximations in the paper with a quick scan.
First is the assumption that standard deviation can be applied. As I
remember form 40 years ago, the concept for standard deviation only applies
to distributions that are Gaussian. They make no proof or present any
evidence as to just what is the error distributions and why they should be
random with a Gaussian distribution. The phenomena that is causing the
variation could be defined and it is entirely possible that the errors are
Gaussian, but I rather doubt it.

Taking the average if a few things can be very misleading and fraught with
statistical errors. If you run some basic probability test like Student T
(again only applicable for Gaussian distributions as I recall) or what ever
to see just what confidence there is that the average is any more accurate
than any one of the cells.

A very good example that all can appreciate is that of taking the reading of
time from 6 watches and averaging them and expecting to get more accuracy in
knowing what time it is. Here is a case where it is obvious that the error
is not random but arbitrary, a really quite different thing. It should be
obvious the average reading of the watches is no more accurate than any one
of the watches as you do not have a clue as to the distribution of the
errors or their cause, except the are not random, just different. Now if you
have 10,000 watches, you have another hunk of mathematics coming in to play
and that is the central limit theorem that says if you got enough
independent samples of any distribution they will appear Gaussian and all
the good math applies. Six is not a large enough to qualify under this
caveat.

Another way to see the error of this thinking is to make a simple
assumption. One of the 5 cells is dead ass on the correct voltage and the
other four are wrong. The average will also be wrong. just randomly picking
one of the 5 cells for the standard will make you right 20% of the time
whereas the average will make you wrong 100% of the time.

WBob

----- Original Message ----- 
From: "Tom Dawson" <wb3akd at earthlink.net>
To: <test-equipment at mailman.qth.net>
Sent: Monday, May 31, 2004 8:59 AM
Subject: [Test-Equipment] Corrected link


> Sorry,
>
> Fumble fingered the previous post.
>
>
> Follow the link to the following NBS Tech Note relating to measurements of
> standard cells, and maintaining the volt with a small group of saturated
> standard cells.
>
>
> http://www.radio-science.com/std_cells/tn_430.pdf
>
> and a dos program to do the arithmetic.
>
> http://www.radio-science.com/std_cells/cells.exe
>
>
> regards,
>
> Tom
>
>
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