>>I guess the degradation which is found would come from parameters which
>>are determined by both datasets and come out with different values in each
>>separate refinement.
If they come out differently it is because they are differently biased by
different systematic errors in the data not described by the model.
>Not necessarily. In order to get the ESD, the variance-covariance matrix is
>multiplied by chi^2, and the roots of the diagonal elements are taken.
>Therefore, if the chi^2 of the combined refinement is worse than that of the
>individual ones, the ESD will automatically be worsened.
You may also get a higher chi^2 with higher resolution data, so does that
mean that the structure will be less well determined with hi res data ?
I think not, because the correlation between structural parameters should
then be smaller - even if you have more points to fit with the same number
of parameters, and the peak shapes are less well described by the model.
You should similarly do better if you have both X-ray and neutron data
(in the absence of bias).
>Also, by adding reflections that are insensitive to
>a given parameter my feeling is that you increase the esd on that parameter
>even if chi^2=1, but the proof of this is too tedious.
Tedious and also impossible ? (This sounds like a contradiction in terms)
There is no case you can make based on pure statistics or the mathematics
of refinement. The only way combined refinement can be worse is if you
introduce bias through systematic error (which unfortunately may happen).
>... in most cases that the ESD's are underestimated.
Mainly because the ESD's are only correctly calculated if the model
is CAPABLE of fitting the data. This is not usually true when systematic
errors are important compared to statistical errors, since the model is
usually not capable of describing these systematic errors fully -
background, texture etc...
The conclusion is that you should use combined refinements provided that
one set of data does not contain important uncorrected systematic errors.
Alan Hewat, ILL Grenoble, FRANCE <[EMAIL PROTECTED]> tel (33) 4.76.20.72.13
ftp://ftp.ill.fr/pub/dif fax (33) 4.76.48.39.06 http://www.ill.fr/dif/
