Hello Hao: I took a look at your fits _very_ briefly. Just a couple of things to note. The range of chi(k) which you use in the fit is probably a bit large. You are starting at 2.1 and the data at that point is really not usable. I would start at 3 or so. On the high end, it seems OK. The estimated standard deviations on lots of your parameters are very large, on the order of the parameter values themselves. This is a red flag and it generally indicates that the fit has too many variables. This is the biggest concern that I have. Look at the values: dr = 0.0010050 +/- 0.0111490 ss = 0.0031940 +/- 0.0021850 dr_1 = 0.0464110 +/- 0.1100960 ss_1 = 0.0130090 +/- 0.0188820 dr_2 = 0.0896690 +/- 0.0765040 ss_2 = 0.0043400 +/- 0.0106960 n1 = 3.3690270 +/- 3.5237430 most of these values are indistinguishable from zero. (As an aside, are you getting an error box when you try to fit? The parameter "dr" is not allowed in newer versions of Artemis.) Here is my suggestion: Change the chi(k) range to 3-13.2. Change the fitting range to 1-2.65 or so. Link the ss_1 and ss_2 parameters together and set dr to 0.0 as it is basically there already and has a large uncertainty. By doing this you will have reduced significantly the number of parameters and the uncertainties will be more manageable. More importantly, when you work on a model, start with a more highly constrained one (fewer paths, for example) and try to work your way up while always making sure that the results make sense. You will be able to reduce the parameter space that you have to cover this way. Keep a close eye on the "Reduced Chi-square" as you add more "guess" parameters. If it goes up, you may be overreaching on your model even though the "R-factor" goes down! HTH Carlo -- Carlo U. Segre -- Professor of Physics Associate Dean for Special Projects, Graduate College Illinois Institute of Technology Voice: 312.567.3498 Fax: 312.567.3494 segre@iit.edu http://www.iit.edu/~segre segre@debian.org