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Re: [ccp4bb] Second moments of Z but not twinning
 

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CCP4bb <-- 2007 <-- June 2007 <-- 01 June 2007
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Subject: Re: Second moments of Z but not twinning
From: Eleanor Dodson ccp4 {- at -} YSBL {- dot -} YORK {- dot -} AC {- dot -} UK
Date: 2007-06-01 		Next message:
Subject: Re: Douglas Instruments Oryx crystallisation robots
From: Jens-Christian Navarro Poulsen jnp {- at -} CCS {- dot -} KI {- dot -} KU {- dot -} DK
Date: 2007-06-01


Subject: Re: Second moments of Z but not twinning
From: David Briggs bassophile {- at -} GMAIL {- dot -} COM
Date: 2007-06-01
Hi Graeme,

How is your data binned?

I don't claim to be any statistician, but I seem to remember being told by
such an individual that you need a decent number of reflections to be able
to calculate a accurate/reliable second moment - and if, in your low-res
bins, you have relatively few reflections, your values of second moment of Z
can start to vary wildly.

HTH,

Dave

On 01/06/07, Winter, G (Graeme) wrote:
>
> Hi Folks,
>
> I was wondering if there is a document somewhere explaining some of the
> more interesting things which can be interpreted from the plot of second
> moment of Z = I'/ anywhere? I've looked around and I can't find much
> beyond the things pertaining to twinning, where it will have the magical
> values of 2.0 or 1.5. What about if it is say 2.0 around the low
> resolution regions but gets much bigger?
>
> Looking at the truncate source code there is a note to the effect that the
> second moments may be greater than two in when you have pseudotranslational
> symmetry. In this case, however, I would expect that the effect would be
> uniform across the resolution ranges… any thoughts?
>
> Any pointers would be greatly appreciated.
>
> Thanks,
>
> Graeme
>



--
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David Briggs, PhD.
Father & Crystallographer
www.dbriggs.talktalk.net
iChat AIM ID: DBassophile
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