I’m using a TMultiGraph made of several TGraphErrors and I’ve fitted it with a TF1 user defined function: +*x^.
The initial fit wasn’t working but after I’ve limited  and  I’ve got a fit function which works perfectly good, the ratio to data being almost 1.
Now, my problem is that even if the fit works this good, the fit parameters have quite large errors and the offset  isn’t corresponding to data points.
This is what I get:
FCN=3.11353 FROM MIGRAD STATUS=CONVERGED 190 CALLS 191 TOTAL EDM=1.07763e-06 STRATEGY= 1 ERROR MATRIX ACCURATE EXT PARAMETER STEP FIRST NO. NAME VALUE ERROR SIZE DERIVATIVE 1 p0 -2.09829e-01 8.02080e-01 9.57594e-06 -1.12207e-01 2 p1 5.83547e-01 6.93479e-01 3.13843e-06 -3.83076e-01 3 p2 5.49078e-01 4.21060e-01 4.47777e-06 -2.98546e-01 Also, another big problem is this: PARAMETER CORRELATION COEFFICIENTS NO. GLOBAL 1 2 3 1 0.99993 1.000 -0.999 0.997 2 0.99997 -0.999 1.000 -0.999 3 0.99986 0.997 -0.999 1.000
What should I do in this case for better estimation of the parameters and of their errors?
I’ve understood that it is possible that the fit function doesn’t take into account the errors on X, but I haven’t found a solid answer in other posts.
Thank you in advance,