While it's hard to argue with Wikipedia (since it's 100% reliable,
etc etc...), here is my two ha'porth.
By the way, in the next paragraph read "multiply" as the adverb and
not the verb.
It's hard to find real evidence for this, but my thought was that the
use of "redundant" in cases like this is a short-hand way of saying
"multiply redundant" - the system where multiple systems are
installed which are individually truly redundant. Multiple redundancy
is particularly popular in critically important systems like manned
spacecraft. In a RAID system, of course, each redundant disk is truly
redundant - if a single disk fails, your data are still there (you
just have to think about getting a replacement hard drive).
My guess is that people have just dropped the "multiply" bit and
carried on with the "redundant". There are plenty of other examples
around, e.g. in the UK we talk about "football" while dropping the
"Association" or "Rugby" (depending on the shape of your ball); in
the States they shudder at both of these (in Australia I understand
they have a game where they drop the "football" and just call their
own version "Aussie Rules", while many people across the Globe are
happy to drop the "Football" altogether and call the Beautiful Game
"Soccer".
We also (as crystallographers all around the world, not just on both
sides of the pond) talk about "freezing" our crystals and collecting
"reflections". Both are wrong, but our peers understand what we mean.
On 14 May 2013, at Tue14 May 23:25, Ethan Merritt wrote:
On Tuesday, May 14, 2013 01:58:06 pm Colin Nave wrote:
The use of the term redundancy (real or otherwise!) in
crystallography
is potentially misleading as the normal usages means superfluous/
surplus
to requirements.
That may be true in the UK, but on this side of the pond "redundancy"
normally refers to ensuring a safety margin by having more of whatever
than is strictly needed for functionality, so that even if some of the
whatsits fail you have enough remaining to go on with. The use of
the term in crystallography is perfectly normal to American ears.
Here's a definition from Wikipedia
"redundancy is the duplication of critical components or functions
of a system with the intention of increasing reliability of the
system..."
just another tidbit of cross-pond difference in language.
Ethan
The closest usage I can find from elsewhere is in information
theory where it is applied for purposes of error detection when
communicating over a noisy channel. Seems similar to the
crystallographic use.
The more relevant point is what sort of errors would be mitigated
by having different paths through the crystal. The obvious ones
are absorption errors and errors in detector calibration. Inverse
beam methods can mitigate these by ensuring the systematic errors
are similar for the reflections being compared. However, my
interpretation of the Acta D59 paper is that it is accepted that
systematic errors are present and, by making multiple measurements
under different conditions, the effect of these systematic errors
will be minimised.
Can anyone suggest other sources of error which would be mitigated
by having different paths through the crystal. I don't think
radiation damage (mentioned by several people) is one.
Colin
From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On Behalf
Of Frank von Delft
Sent: 14 May 2013 14:23
To: ccp4bb
Subject: [ccp4bb] Fwd: Re: [ccp4bb] reference for "true
multiplicity"?
George points out that the quote I referred to did not make it to
the BB -- here we go, read below and learn, it is a most succinct
summary.
phx
-------- Original Message --------
Subject:
Re: [ccp4bb] reference for "true multiplicity"?
Date:
Tue, 14 May 2013 09:25:22 +0100
From:
Frank von Delft <frank.vonde...@sgc.ox.ac.uk><mailto:%
3cfrank.vonde...@sgc.ox.ac.uk%3e>
To:
George Sheldrick <gshe...@shelx.uni-
AC.GWDG.DE><mailto:gshe...@shelx.uni-ac.gwdg.de>
Thanks! It's the Acta D59 p688 I was thinking of - start of
discussion:
"The results presented here show that it is possible to solve
protein structures using the anomalous scattering from native
S atoms measured on a laboratory instrument in a careful but
relatively routine manner, provided that a sufficiently high
real redundancy is obtained (ranging from 16 to 44 in these
experiments). Real redundancy implies measurement of
equivalent or identical re�ections with different paths through
the crystal, not just repeated measurements; this is expedited
by high crystal symmetry and by the use of a three-circle (or )
goniometer."
Wise words...
phx
On 14/05/2013 08:06, George Sheldrick wrote:
Dear Frank,
We did extensive testing of this approach at the beginning of this
millenium - see
Acta Cryst. D59 (2003) 393 and 688 - but never claimed that it was
our idea.
Best wishes,
George
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
--
Ethan A Merritt
Biomolecular Structure Center, K-428 Health Sciences Bldg
University of Washington, Seattle 98195-7742
Harry
--
** note change of address **
Dr Harry Powell, MRC Laboratory of Molecular Biology, Francis Crick
Avenue, Cambridge Biomedical Campus, Cambridge, CB2 0QH
Chairman of European Crystallographic Association SIG9
(Crystallographic Computing)
