Hi Mailing list, Well, the chief concern I have with my fit now is that I have some negative parameters (delr and e0_1): Guess parameters +/- uncertainties (initial guess): amp = 0.7709790 +/- 0.0631820 e0 = 1.1165150 +/- 1.0100360 delr = -0.0106440 +/- 0.0077110 ss = 0.0032510 +/- 0.0005200 e0_1 = -4.6848070 +/- 1.7781210 third = 0.0000370 +/- 0.0001070 So, I guess I can drop the third cumulant. This is a fit using 6 paths (1,2,3,6 are single scatterers) These are the results this time around: Reduced Chi-square = 410.285252396 R-factor = 0.014599348 As you can see I am still using two e0's, it is still dramatically improving the fit. Without: Reduced Chi-square = 1133.485950636 R-factor = 0.044551155 any advice welcomed, Dan