On Wednesday 02 September 2009 04:13:55 am Jens Kruse wrote:
Hi,
I have only two short questions. Is it possible that values for chi-square and reduced chi-square given in the "Fit results" and the "Combinatorics" page are inverted within Athena?
I tried running a combinatorial LCF series just before writing this email and I think these numbers are reported correctly. If you think otherwise, you will need to show me an example. Of course, the values for both R-factor and reduced chi-square are rather peculiar in Athena's LC fitting, as explained in the manual: http://cars9.uchicago.edu/~ravel/software/doc/Athena/html/analysis/lcf.html
In the CSV report for all fits, which of these both columns ("chisqr", "chinu" ) is chi-square and which is reduced chi-square?
I don't think these are reported wrongly. As I discuss in the manual, it is hard to know how to evaluate chi-nu in a XANES fit. I use epsilon=1 (for want of a better number) and I use the number of points in the data as the number of independent measurements. There are typically of the order of 100 points in a XANES scan, so chi-nu tends to be about 2 orders of magnitude smaller than chi-square, which itself is very small due to the value of epsilon. All of this just underscores the squishiness of the statistical parameters in this context. Interpreting LCF results requies the use of prior knowledge and a very conservative interpretation of what Athena reports. B -- Bruce Ravel ------------------------------------ bravel@bnl.gov National Institute of Standards and Technology Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973 My homepage: http://xafs.org/BruceRavel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/