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: [log in to unmask]
Cambridge CB2 0XY, U.K. www-structmed.cimr.cam.ac.uk
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