Kurt (and others), Intensity in cps isn't a problem if the conversion is also applied to the esd and both intensity and esd is output to be read by GSAS. The problem is that the instrument software doesn't give you the esds at all but just a table (in the "old" 10 column format) of the "intensities" as cps. GSAS then assumes these are counts and that esd = sqrt(counts) according to the usual Gaussian statistics. So chi**2 will be off by the reciprocal of the count time per step, i.e. too small. BTW this does not affect the resulting parameters (or esds) one bit - it only makes the chi**2 "wrong". BTW**2: Poisson statistics only comes into play when the count is less than ~20 so that zero is within (say) 4-5 sigma of the count. Above that Gaussian statistics ~ Poisson statistics. So if all points in the scan have >20 counts (including background) then the assumptions about Gaussian statistics for the counts are reasonable. BTW**3: If the background was say 20 counts with a sig ~4-5 then the "R-! value" for that background will be ~20%. So if your pattern has very few peaks (say only 10% of the points) then R-values can easily be much larger than 10%. Following this logic a bit further, a pattern with a higher background will give lower R-values so IMHO comparing R-values from one experiment to another is a particularly pointless exercise. Ditto for asking "how good does my R-factor have to be?" because it just depends. Bob
-----Original Message----- From: Kurt Leinenweber [mailto:[EMAIL PROTECTED] Sent: Tue 2/19/2008 10:37 AM To: rietveld_l@ill.fr Subject: RE: advice on new powder diffractometer Hi Bob, The band of delt/sig is a little wider than that, but I think the real problem is that they sent me the data in cps. I am awaiting the data in counts now. Thanks to you and others for pointing this out. However, in my view, having the data in cps should simply go into the histogram scale factor and should not affect the fit. What is wrong with my logic? Thanks, - Kurt -----Original Message----- From: Von Dreele, Robert B. [mailto:[EMAIL PROTECTED] Sent: Friday, February 15, 2008 6:37 AM To: rietveld_l@ill.fr Subject: RE: advice on new powder diffractometer Hi Kurt, If you plot the results in POWPLOT & do the "statistical analysis" what does the delt/sig vs 2-theta plot look like? If weights are right then the band of delt/sig ought to be roughly 2-3 sig wide. Is your data in cpm? Some instruments make that by default. That will mess up the R-factors. Bob
