No, simply applying a single overall scale factor to the intensities can't possibly make any difference to the Wilson B since the fall-off with resolution will remain unchanged. The Wilson plot is a plot of ln(mean(I')/S) in shells of constant d* vs d*^2, where I' is I corrected for symmetry and S is a function of the scattering factors for the known unit cell content. Changing the overall scale factor shifts the plot up or down but doesn't change the gradient, and the Wilson B factor depends on the gradient (actually B = -2*gradient).
In any case detwinning is impossible if as you say the twin fraction is near 0.5. Your procedure doesn't perform detwinning. For example, suppose the true intensities of the components of the twin are (say) 90 and 110. For tf = 0.5 you will observe the mean value (i.e. half from each component), so I(twin) = 100. Taking I(twin)/2 = 50 doesn't give you back the true intensity (in fact in this case I(twin) is actually a better estimate of I(true)); in any case any attempt at detwinning must give you 2 values, one for each component of the twin. Cheers -- Ian On Fri, May 20, 2011 at 3:43 PM, fulvio saccoccia <fulvio.saccoc...@uniroma1.it> wrote: > 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 >> > > > >
