If the model is really bad and sigmaA is estimated properly, then sigmaA will be close to zero so that D (sigmaA times a scale factor) will be close to zero. So in the limit of a completely useless model, the two methods of map calculation converge.
Regards, Randy Read On 11 Oct 2011, at 19:47, Ed Pozharski wrote: > On Tue, 2011-10-11 at 10:47 -0700, Pavel Afonine wrote: >> better, but not always. What about say 80% or so complete dataset? >> Filling in 20% of Fcalc (or DFcalc or bin-averaged <Fobs> or else - it >> doesn't matter, since the phase will dominate anyway) will highly bias >> the map towards the model. > > DFc, if properly calculated, is the maximum likelihood estimate of the > observed amplitude. I'd say that 0 is by far the worst possible > estimate, as Fobs are really never exactly zero. Not sure what the > situation would be when it's better to use Fo=0, perhaps if the model is > grossly incorrect? But in that case the completeness may be the least > of my worries. > > Indeed, phases drive most of the model bias, not amplitudes. If model > is good and phases are good then the DFc will be a much better estimate > than zero. If model is bad and phases are bad then filling in missing > reflections will not increase bias too much. But replacing them with > zeros will introduce extra noise. In particular, the ice rings may mess > things up and cause ripples. > > On a practical side, one can always compare the maps with and without > missing reflections. > > -- > After much deep and profound brain things inside my head, > I have decided to thank you for bringing peace to our home. > Julian, King of Lemurs ------ Randy J. Read Department of Haematology, University of Cambridge Cambridge Institute for Medical Research Tel: + 44 1223 336500 Wellcome Trust/MRC Building Fax: + 44 1223 336827 Hills Road E-mail: rj...@cam.ac.uk Cambridge CB2 0XY, U.K. www-structmed.cimr.cam.ac.uk
