Error of fitting curves

Dear all,

I have been trying to fit some data points with a straight line (as shown in the attached plot). The points were fitted against [0]*x + [1] and I got parameter [0] = -1E-03. However, the error I got for this parameter is 5.04E-04 which is 50% of the parameter!! But as shown in the plot, the fitted straight line goes through all the points and it seems that the data points are fitted beautifully, thus I do not understand why the error is so large. Can anyone explain ROOT’s fitting algorithm and why the error is so large?

Thank you in advance for your help!


If your data points have a large errors your fit, as shown in the figure, your fit will give large errors in the parameters, you can’t avoid that.

The fact that the data are such in good agreement with the straight line
(the chi2 resulting from the fit is very low), indicates that most probably your data error are over-estimated. You should check them,

Best Regards,