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: [email protected] Tel: ++972-3640-8723 Fax: ++972-3640-9407 Cellular: 0547 459 608 On May 14, 2013, at 11:19 , Tim Gruene <[email protected]> wrote: > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 > > 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 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 >> > > - -- > - -- > Dr Tim Gruene > Institut fuer anorganische Chemie > Tammannstr. 4 > D-37077 Goettingen > > GPG Key ID = A46BEE1A > > -----BEGIN PGP SIGNATURE----- > Version: GnuPG v1.4.12 (GNU/Linux) > Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ > > iD8DBQFRkfN7UxlJ7aRr7hoRAnlWAJ9T4MvGHUGA+HRwOL2i/6rU7KW1xwCcDsAq > KAvPG9FqtNYO2kLqmh7wIZI= > =MNNU > -----END PGP SIGNATURE-----
