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 >
