# Evaluate the quality of the fit ?

Posted 1 month ago
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 Hi, I performed the fit of my data and I would like to obtain various information concerning the quality of the fit. Curiously, Mathematica doesn't respond as it normally would. Here is the code used Clear[k3, k4, ea, eh, ef] sol = ParametricNDSolve[{Derivative[1][a][t] == -k3*a[t], Derivative[1][h][t] == k3*a[t] - 2*k4*h[t]*h[t], Derivative[1][f][t] == 2*k4*h[t]*h[t], a[0] == 0.000006, h[0] == 0, f[0] == 0}, {a, h, f}, {t, 0, 2000}, {k3, k4}]; {aFunc, hFunc, fFunc} = {a, h, f} /. sol; model2[k3_, k4_, ea_, eh_, ef_][ t_] := ((aFunc[k3, k4][t]*ea) + (eh*hFunc[k3, k4][t]) + (ef* fFunc[k3, k4][t]))*100/aFunc[k3, k4][0]; fit1 = FindFit[ dataabs380, {model2[k3, k4, ea, eh, ef][t], 0.00050 < k3 < 0.02, 700 < k4 < 1200, 0.0001 < ea < 0.0008, 0.0001 < eh < 0.0008, 0.0001 < ef < 0.0008}, {k3, k4, ea, eh, ef}, t]; fit1["ParameterTable"] fit1["ParameterErrors"] The output for Fit["ParameterTable"] is Ok. It leads to {k3 -> 0.0085098, k4 -> 912.36, ea -> 0.000360078, eh -> 0.000316874, ef -> 0.000559417}["ParameterTable"] But the output for Fit["ParameterErrors"] is similar to the previous one. {k3 -> 0.0085098, k4 -> 912.36, ea -> 0.000360078, eh -> 0.000316874, ef -> 0.000559417}["ParameterErrors"] I tried several others options available to estimate the quality of the fit, but none works. I don't understand this strangeness. It is not a problem of syntax.Thanks in advance for all your advice and help. Sincerely Helene
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Posted 1 month ago
 Hi Helene,FindFit does not return a FittedModel, try using NonlinearModelFit.