Re: [sdpd] error measurements

[cockcroft...@gordon.cryst.bbk.ac.uk]

In reply to Jaap's last comment,
  You can subtract the background in PRACTICE too after refinement as 
long as you know the function used for the refinable background (e.g. 
the polynomial coefficients when a polynomial is used.) This can
be done each cycle of refinement (and is trivial to code). Whether
the peaks overlap or not is irrelevant - or even whether you can see
the background between peaks.
  After each cycle of refinement, simply do
     yi(calc. diffraction)' = yi(calc. total) - yi(calc. background)
and  yi(obs. diffraction)' = yi(obs. total) - yi(obs. background)
and then use yi' values for calculating Rwp, Rexp, etc. At least Rwp then
bears a better relationship to the structural side of the refinement.
  Whether the commonly used codes do this or not is another matter. But
just because some software doesn't calculate the "correct" numbers isn't
a justification (IMHO) for publishing "wrong" ones. (Obviously the
words in quotes are an my opinion only.)
  Jeremy