On Monday 12 July 2004 03:25 pm, Scott Calvin wrote:
Having said that, I'd love a discussion as to how people obtain meaningful estimates of epsilon, or under what conditions they consider the Ifeffit default good enough.
As you noted, it is worth reiterating that the algorithm Ifeffit uses to find uncertainties in fitted parameters is independent of the choice of epsilon.
I would emphasize that last point. The error bars on parameters are always reasonable because they are computed by scaling the diagonal elements of the covarience matrix by the square root of chi-square. That is *the same thing as* reeveluating the covarience matrix with the value of epsilon that results in a reduced chi square of 1. Thus, if you believe that the fit is a good fit, the error bars are the right size. As far as the actual value of epsilon -- I am not sure it is really all that important. There is plenty of information available to your to evaluate a fit. The R-factor is a start, as are the error bars and ones physical intuition about the system. The use of reduced chi-square is to compare two different models. If you change a fitting model in some way, you expect the reduced chi-square to be sufficiently smaller for the better fit. If reduced chi-square doesn't change enough, then the two fitting models are statistically indistinguishable. Another way of saying that is that the R factor is a useful metric for deciding if THIS fit is reasonable. The reduced chi-square is a useful metric for deciding if THIS fit is better than THAT fit. As to whether you need to report reduced chi-square in a paper, I agree with Matt. If it's an important part of the story, it's an important part of the paper. But if you include it in the paper, you had better be prepared to explain the previous paragraph and to explain why it is so hard to estimate epsilon. Or at least to refer to a paper which explains those things to your satisfaction. B -- Bruce Ravel ----------------------------------- ravel@phys.washington.edu Code 6134, Building 3, Room 405 Naval Research Laboratory phone: (1) 202 767 2268 Washington DC 20375, USA fax: (1) 202 767 4642 NRL Synchrotron Radiation Consortium (NRL-SRC) Beamlines X11a, X11b, X23b National Synchrotron Light Source Brookhaven National Laboratory, Upton, NY 11973 My homepage: http://feff.phys.washington.edu/~ravel EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/