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OriginPro 8G SR2 v8.0891 (B891)
I got a problem with the nonlinear curve fitting. I use the formular:
dependent variable y
independent variable x
fit parameters a, b
y=log(a*(0.254829592*1/(1+0.32759*x/b)-0.284495736*(1/(1+0.32759*x/b))^2+1.421413741*(1/(1+0.32759*x/b))^3-1.453152027*(1/(1+0.32759*x/b))^4+1.061405429*(1/(1+0.32759*x/b))^5)*exp(-x/b*x/b))
which is a erfc function. The fitting plot realy fits (r^2>0.99) my data but the strange thing is that the calculated variables are wrong in comparison to the values easyplot, igor and different data analyser calculate (I fitted the data several times with a few analyser with different guesses for a,b and just origin calculate different values). The origin values for "a" are around 20 and should be 400-600. The values for "b" are around 2200 and should be 1400.
I really do not understand how this can happen. I hope somebody can help me. If necessary i could send the data and the fdf file. I would really like to use origin as my analyser.
Thanks a lot |
nonlinear fitting problems
Posted - 04/09/2009 : 07:35:53 AM
