Hello all, A quick question regarding the background subtracted R values:
In the .LST file, GSAS gives two sets of values for wRp and Rp: "Fitted" and "- Bknd." I understood the Fitted values to be calculated including the background, and the -Bknd to be calculated with the background removed from both the calculated and observed patterns. Is this correct? I notice in some refinements I am doing that the - Bknd values are smaller than the Fitted values which is not what I would have expected. Thanks in advance, Matt ------------------------------------------------------------------ Matt Beekman Department of Physics University of South Florida 4202 East Fowler Ave., PHY 114 Tampa, FL 33620 Phone: (813) 974-8236 Fax: (813) 974-5813 Email: mbeek...@cas.usf.edu Visit the Novel Materials Laboratory Website: http://chuma.cas.usf.edu/~gnolas/<https://frontend.cas.usf.edu/exchweb/bin/redir.asp?URL=http://chuma.cas.usf.edu/~gnolas/> On Fri, Mar 6, 2009 at 2:55 PM, Brian H. Toby <brian.t...@anl.gov> wrote: > I am not sure what all the fuss on background subtraction is all about, as > long as the uncertainties in the data are unchanged. We have to model > background in all powder methods and add it to the computed pattern. If you > subtract something from the obs pattern before you fit, the only things that > changes is the normalization term in the R factors. Since the purpose of the > weighted R-factor is to measure the quality of different models to the same > data, this really makes no difference. The unweighted r-factor is pretty > much meaningless, so again who cares? > I don't love the idea of modifying data, rather than applying a correction > to a calculated value -- observations should be sacrosanct, IMHO, but having > said that, more than once have I applied an absorption correction directly > to measured intensities. > > The important point is that if you measure 100 counts at a point, the > uncertainty in that is 10. If you decide that 95 of those counts are > background, go ahead and subtract them if you must, but the uncertainty on > the observation remains 10, and does not drop to sqrt(5) -- this means you > must compute uncertainties before subtracting and use a Rietveld code that > will accept "xye" input, as Pam pointed out. > > Brian > > ******************************************************************** Brian > H. Toby, Ph.D. office: 630-252-5488 > Senior Physicist/Materials Characterization Group Leader > Advanced Photon Source > 9700 S. Cass Ave, Bldg. 433/D003 work cell: 630-327-8426 > Argonne National Laboratory secretary (Marija): 630-252-5453 > Argonne, IL 60439-4856 e-mail: brian dot toby at anl dot gov > ******************************************************************** > "We will restore science to its rightful place, and wield technology's > wonders... We will harness the sun and the winds and the soil to fuel our > cars and run our factories... All this we can do. All this we will do." > >
