On Wednesday, October 12, 2011 02:35:21 pm Matt Newville wrote:
I used to quote in my papers that typical statistical and systematic errors lead to a total error in first shell distances of +-0.02 A. Quoting +/- 0.02 Ang seems like a reasonable estimate for a typical value, but that's hardly rigorous.
I don't have a better answer for Riti's question than the one Matt gave. I do, however, want to get up on a soapbox about the practice of quoting typical error bars. While I certainly agree that something like 0.02 is an unsurprising uncertainty in a distance, there is a very good reason to always report measured uncertainties. The problem with sweeping measured uncertainties under the rug by asserting a typical value is that it is probably not true that for a real-world, interesting, research project that *all* the uncertainties are of a typical size. Because of issues of experimental error and correlation between parameters, it is likely that some of your parameters have uncertainties that are larger than the typical value. You may even have some that are smaller. It is probably also true that some of the most interesting parameters in the fit -- the ones that are most salient to the argument being made in the manuscript -- are the ones with non-typical uncertainties. If you fail to report that part of your analysis, you are depriving your reader of information that is important to the evaluation of your results. It is for that reason that I discourage the practice of citing typical uncertainties. B P.S. This paper by Corwin Booth about error analysis was one of my favorite presentations at the last XAFS conference http://dx.doi.org/10.1088/1742-6596/190/1/012028 -- 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/ifeffit/Demeter