> On 10/11/11 12:58, Ethan Merritt wrote:
> > On Tuesday, October 11, 2011 12:33:09 pm Garib N Murshudov wrote:
> >> In the limit yes. however limit is when we do not have solution, i.e. when
> >> model errors are very large. In the limit map coefficients will be 0 even
> >> for 2mFo-DFc maps. In refinement we have some model. At the moment we have
> >> choice between 0 and DFc. 0 is not the best estimate as Ed rightly points
> >> out. We replace (I am sorry for self promotion, nevertheless: Murshudov et
> >> al, 1997) "absent" reflection with DFc, but it introduces bias. Bias
> >> becomes stronger as the number of "absent" reflections become larger. We
> >> need better way of estimating "unobserved" reflections. In statistics
> >> there are few appraoches. None of them is full proof, all of them are
> >> computationally expensive. One of the techniques is called multiple
> >> imputation.
> >
> > I don't quite follow how one would generate multiple imputations in this
> > case.
> >
> > Would this be equivalent to generating a map from (Nobs - N) refls, then
> > filling in F_estimate for those N refls by back-transforming the map?
> > Sort of like phase extension, except generating new Fs rather than new
> > phases?
> >
> > Ethan
Dale Tronrud wrote>
>
> Unless you do some density modification you'll just get back zeros for
> the reflections you didn't enter.
Sure. And different DM procedures would give you different imputations,
or at least that was my vague idea.
Garib N Murshudov wrote>
> Best way would be to generate from probability distributions derived after
> refinement, but it has a problem that you need to integrate over all errors.
> Another, simpler way would be generate using Wilson distribution multiple
> times and do refinement multiple times and average results. I have not done
> any tests but on paper it looks like a sensible procedure.
OK. That makes sense.
Ethan
--
Ethan A Merritt
Biomolecular Structure Center, K-428 Health Sciences Bldg
University of Washington, Seattle 98195-7742
