Hi List, It appears that changing the k-weight on my fit does improve the fit. I still have the problem of forcing the sO^2's on the two MS paths to be positive. Here's some new data with k-weight 3: Thanks, Dan Project title : Fitting merge 10_15_04.chi ============================================================ Independent points = 28.581054688 Number of variables = 13.000000000 Chi-square = 1527.118835332 Reduced Chi-square = 98.011262136 R-factor = 0.025169471 Measurement uncertainty (k) = 0.000185138 Measurement uncertainty (R) = 0.081594539 Number of data sets = 1.000000000 Guess parameters +/- uncertainties (initial guess): e0 = 1.1335810 +/- 1.3393990 (guessed as 1.133581 (1.339426)) ss = 0.0035870 +/- 0.0003110 (guessed as 0.003587 (0.000311)) ss2 = 0.0042850 +/- 0.0002780 (guessed as 0.004285 (0.000278)) ss3 = 0.0303100 +/- 0.0510420 (guessed as 0.030310 (0.051043)) ss4 = 0.0000000 +/- 0.0303840 (guessed as 0.000000 (0.030385)) ss5 = -0.0008090 +/- 0.0043530 (guessed as -0.000809 (0.004353)) ss6 = 0.0059560 +/- 0.0089810 (guessed as 0.005956 (0.008981)) alpha= -0.0006040 +/- 0.0023740 (guessed as -0.000604 (0.002374)) alpha2= 0.0073900 +/- 0.0016520 (guessed as 0.007390 (0.001652)) alpha3= -0.0167250 +/- 0.0535940 (guessed as -0.016725 (0.053595)) alpha4= -0.0120340 +/- 0.0773040 (guessed as -0.012034 (0.077302)) alpha5= -0.0216900 +/- 0.0127530 (guessed as -0.021690 (0.012751)) alpha6= 0.0219980 +/- 0.0183950 (guessed as 0.021998 (0.018395)) Def parameters: delr_1 = -0.0027560 delr_2 = 0.0337230 delr_3 = -0.0763210 delr_4 = -0.0549150 delr_5 = -0.0989780 delr_6 = 0.1003830 Set parameters: amp1 = .7 Correlations between variables: e0 and alpha --> 0.8490 e0 and alpha2 --> 0.8006 ss3 and alpha4 --> 0.7123 alpha and alpha2 --> 0.6792 ss4 and alpha5 --> 0.6298 ss and ss2 --> 0.6114 alpha5 and alpha6 --> -0.5959 ss5 and alpha4 --> 0.5328 ss3 and ss4 --> 0.5300 ss5 and ss6 --> -0.4931 ss4 and alpha3 --> -0.4617 ss2 and alpha5 --> 0.3847 ss6 and alpha6 --> 0.3700 ss2 and alpha3 --> 0.3276 ss4 and ss5 --> -0.2764 alpha2 and alpha4 --> 0.2506 All other correlations are below 0.25