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<div>Greeting RASMBers -<br>
<br>
Here is a puzzling little matter on which comments are invited. It's
hardly a new point being made, so much as an old one wrapped up in AUC
paper.<br>
<br>
We all know that the cheerful assumption that errors in measured
quantities are normally distributed is a hazardous one. In the extreme
case of 'rounding error' (in computational work) a simple range
defines the region in which values may lie, as compared to a
bell-shaped curve with (increasingly less likely) values going off to
infinity either side. For physical instrumentation with quality
control defining limits, a bell-shaped curve is also unlikely to be a
good representation of distribution of errors.<br>
<br>
Now, the temperature control in an AUC is the most important factor
limiting the estimation of absolute s values, its magnitude equalling
all other factors put together (Errington & Rowe, 2003). The
Beckman XL instruments have a stated accuracy of ±0.5º in
temperature. For which assurance one devoutly trusts there is
evidence, with a QA backing. In other words, we can be confident that
if we accept the actual temperature as being equal to that stated,
then we will not be more than 0.5º in error. Walter's temperature
measurement method does not suggest otherwise, although obviously a
significant number of XLs was not sampled.<br>
<br>
So here is the question. We have 3 XL's running here, and the range in
s values when we estimate s for the identical samples in different
instruments suggests that two of them differ by an amount which is
approaching 1º. Which is perfectly consistent with the machine spec.
Bad news? Well, maybe not. If we accept that ±0.5º as an outer
limit, then for each estimate from one XL we can<i> discount</i> that
half of the potential variation which lies outside the bounds
specified by the other estimate. Indeed, if we push this argument to
its limit, we can discard all values for the 'true' temperature
which lie at all significantly away from the mean of the two
values!<br>
<br>
So - is this a paradox? Given the type of error distribution expected,
can we really accept that it is better, using 2 instruments, to get 2
estimates for an s value that<u> dis</u>-agree than 2 which agree? Or
is this believing in Santa Claus?<br>
<br>
Regards to all<br>
<br>
Arthur<br>
<br>
N Errington, A J Rowe (2003) "Probing conformation and
conformational change in proteins is optimally undertaken in relative
mode" European Biophysics Journal 32 (5) 511-517<br>
--<br>
*******************************************************<br>
Arthur J Rowe<br>
Professor of Biomolecular Technology<br>
NCMH Business Centre<br>
University of Nottingham<br>
School of Biosciences<br>
Sutton Bonington<br>
Leicestershire LE12 5RD UK<br>
<br>
Tel: +44 (0)115 951 6156<br>
+44 (0)116
271 4502<br>
Fax: +44 (0)115 951 6157<br>
email: arthur.rowe@nottingham.ac.uk<br>
Web:
www.nottingham.ac.uk/ncmh/b<span
></span>usiness<br>
*******************************************************<br>
<br>
<br>
--<br>
*************************<br>
Arthur Rowe<br>
Lab at Sutton Bonington<br>
tel: +44 115 951 6156<br>
fax: +44 115 951 6157<br>
*************************<br>
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