On Thursday 02 October 2008 18:06:19 Matt Newville wrote:
Does it? I thought it used data_uncertainty=1 unless the uncertainty was explicitly specified. Does Athena use a value determined from the sigma when merging data, or is something? Should I read the Users Guide more closely?
Hmmmm... perhaps I am confused. Athena uses Ifeffit's minimize function. As you say, the data uncertainty is set to one, so the diagonals of the covarience matrix will be orders of magnitude too small. My understanding is that Ifeffit rescales the error bars on the variable parameters in the same manner as the feffit function does -- that is, by the square root of reduced chi-square. In that case, the error bars are reported are 1-sigma error bars given the assumption that the fit is, in fact, a good fit. Or do I not understand what minimize is doing? Or are there too many possible meanings of the word "sigma" in this context such that we are talking about different things? 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/