[buster-discuss] Calculating the R values for a model at differing resolution cutoffs

[Gerard Bricogne]

Dear Dale and other BUSTER users,

     I am sorry to have left this question unanswered for so long.
     
     The answer is "No", because the R-factors calculated by BUSTER involve
the expectation value of the full "calculated" structure factor under the
same error model that is used to define the likelihood function. This
requires the maximum-likelihood estimation of the Luzzati imperfection
B-factor for the atomic model, and hence an initial ML-scaling step that is
affected by the scope of the experimental data that are used. If you carry
out that step with only low-resolution data, there is no guarantee that you
will get exactly the same result as if you had used higher-resolution data,
then had restricted your attention to a low-resolution subset of those data.

     You can compare that, roughly, to what happens with Wilson scaling: it
works poorly and unpredictably from only low-resolution data, but much
better if you use high-resolution data then restrict the results to their
low-resolution portion.

     I hope this is not too complicated, and not too disappointing.
Comparing things that are intrinsically different always involves using a
statistical model for how they differ (an "error model"), and hence some
estimation of scaling parameters that will depend on those of that error
model.


     With best wishes,
     
          Gerard.

--
On Wed, May 16, 2012 at 12:23:11PM -0700, Dale Tronrud wrote:
> Hi,
> 
>    I am performing refinement with data to one resolution but would like to
> know what the R values are when calculated with only a lower resolution subset
> of the data.  Is there an easy way (I'm lazy) to just calculate the R values
> for an existing model with whatever resolution cutoffs I choose?
> 
> Dale Tronrud
> _______________________________________________
> buster-discuss mailing list
> buster-discuss at globalphasing.com
> https://www.globalphasing.com/mailman/listinfo/buster-discuss

-- 

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