Good morning Colin (from this side of the pond),
I never liked the word redundancy. Multiplicity is a however a good word for multiple measurements. So, Ethan, what does someone in the USA say when made redundant ie out of a job? Surely not that they are now a useful surplus for the US economy of the future? 

Re benefits of multiple measurements I would add:-
Any time dependent related variations such as :-
X-ray beam rapid variations;
Crystal movements;
Variations in cold stream flow;
??
??

More esoterically perhaps extinction for very strong reflections in bigger crystal cases with longer Xray wavelengths. This would be data sets where multiple crystals are needed. This however I don't think affects more than a handful of reflections.

Just my two UK pennies worth,
John


Prof John R Helliwell DSc 
 
 

On 14 May 2013, at 21:58, Colin Nave <[log in to unmask]> wrote:

> Yes, a good summary.
> The use of the term redundancy (real or otherwise!) in crystallography is potentially misleading as the normal usages means superfluous/ surplus to requirements.  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:[log in to unmask]] 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 <[log in to unmask]><mailto:[log in to unmask]>
> 
> To:
> 
> George Sheldrick <[log in to unmask]><mailto:[log in to unmask]>
> 
> 
> 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
> 
> 
> 
> 
> 
> 
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