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 > <fulvio.saccoc...@uniroma1.it> 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 > >
