Thanks Ian,
I tried to do this:
I took the file containing
hkl I and sigI
and generated a new file containing
hkl I/2 and sigI
because I know, from the refined structure that the twin fraction is
nearly 0.5. Now, using this new file the wilson plot give me a more
reliable estimated B factor.
Do you think this procedure was correct?
Fulvio
Il giorno gio, 19/05/2011 alle 14.14 +0100, Ian Tickle ha scritto:
> Hi Fulvio
>
> There are 2 different issues here: the Wilson plot scale & B factor on
> the one hand and Wilson statistics on the other. The first are not
> affected by twinning since they depend only on the intensity averages
> in shells. The second refers to the distribution of intensities (i.e.
> the proportion of reflections with intensity less than a specified
> value) within a shell, or to the distribution of normalised
> intensities (Z = I/<I> ignoring symmetry issues for now) over the
> whole dataset. This distribution is different for a twin because
> averaging the components which contribute to the intensity of a
> twinned reflection tends to shift the distribution towards the mean,
> so you get fewer extreme values.
>
> The Wilson B factor is not a 'statistic' in the strict sense, merely a
> derived parameter. I suspect the low value you get has more to do
> with the fact that the resolution is only 3 A, than the fact it's
> twinned.
>
> See here for more mathematically-oriented info:
>
> http://www.ccp4.ac.uk/dist/html/pxmaths/bmg10.html
>
> Cheers
>
> -- Ian
>
> On Thu, May 19, 2011 at 1:45 PM, fulvio saccoccia
> <[log in to unmask]> wrote:
> > Dear ccp4 users,
> > I have a data set arising from a nearly-perfect pseudo-merohedrally
> > twinned cystal, diffracting up to 3 A. I solved the structure and ready
> > for deposition, but there is still a trouble.
> > The Wilson scaling from raw data gave a B of 3A^2.
> > Initially, I did not seemed too alarming. But I do not know why I have
> > these statistics.
> >
> > Does anyone know why Wilson scaling falls when treating that kind of
> > twinned data? I read that twinned data do not obey twe Wilson statistics
> > but I don't know why.
> > Here the presentation I read:
> >
> > http://bstr521.biostr.washington.edu/PDF/Twinning_2007.pdf
> >
> > Do you know any articles, reviews or book in which this particular
> > aspect of of twinned data is treated in depth, possibly in mathematical
> > manner?
> >
> > Thanks to all
> >
> > Fulvio Saccoccia, PhD student
> > Biochemical Sciences Dept.
> > Sapienza University of Rome
> >
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