It's not about multiplicity, it's about scaling. See quote I sent
earlier. phx.
On 14/05/2013 10:40, Felix Frolow wrote:
I guess that in a standpoint to reduce errors it is easy to improve
statistical errors by longer counting or by using multiple observations.
However the real enemy at the gate is a systematic error which require
special skills and experience to detect and to eliminate.
I never understood why to measure not very good data trying to recover
anomalous signal by improving statistics using very high redundancy
instead of trying
to collect data which are perfect by minimising systematics errors and
of course increasing counting time, but with minimum redundancy of
only 2 ?
Like in good old times with 4 circle diffractometers and good
scintillation counters that produced true counting statistics:
10 counts - 30% precision
100 counts - 10% precision
1000 counts - 3% precision
10000 counts - 1% precision
Canonising and worshipping redundancy looking for "true holy
multiplicity" on my taste is counterproductive…..
My 2 NIS :-)
And of corse - one of the systematic errors is the radiation damage…...
FF
Dr Felix Frolow
Professor of Structural Biology and Biotechnology, Department of
Molecular Microbiology and Biotechnology
Tel Aviv University 69978, Israel
Acta Crystallographica F, co-editor
e-mail: mbfro...@post.tau.ac.il <mailto:mbfro...@post.tau.ac.il>
Tel: ++972-3640-8723
Fax: ++972-3640-9407
Cellular: 0547 459 608
On May 14, 2013, at 11:19 , Tim Gruene <t...@shelx.uni-ac.gwdg.de
<mailto:t...@shelx.uni-ac.gwdg.de>> wrote:
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Hi Frank,
I would not call it 'axiomatic' but 'statistics' to reduce the
(stochastic) error by several independent measurements. You can
probably give any statistics textbook as a reference.
In real life, though, you have to compromise with radiation damage,
though. For references I recommend searching journals.iucr.org
<http://journals.iucr.org> for
'Garman' as author. If you add 'radiation damaga' as keywords, the
result reduces to 37 hits of choice.
Best,
Tim
On 05/14/2013 06:50 AM, Frank von Delft wrote:
Hi, I'm meant to know this but I'm blanking, so I'll crowdsource
instead:
Anybody know a (the) reference where it was showed that the best
SAD data is obtained by collecting multiple revolutions at
different crystal offsets (kappa settings)? It's axiomatic now (I
hope!), but I remember seeing someone actually show this. I
thought Sheldrick early tweens, but PubMed is not being useful.
(Oh dear, this will unleash references from the 60s, won't it.)
phx
- --
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Dr Tim Gruene
Institut fuer anorganische Chemie
Tammannstr. 4
D-37077 Goettingen
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