[ROOT] Errors on fit functions

From: Jason Seely (jason@mit.edu)
Date: Mon Jul 21 2003 - 04:56:37 MEST


hi ROOT people,

i am just wondering if there is an easy way to get the error of a fit. 
 i'm fitting something like this (for example)

    ffgr = new TGraphErrors(imax, x, y, dx, dy);
    ffgr->Fit(func,"","",xmin,xmax);

    TF1    *fitfunc;
    fitfunc = ffgr->GetFunction(func);

so once i have the fitting function, i know how to get parameters, 
chi^2, parameter errors, etc.  what i want is the estimated error on the 
function itself so i can draw an error band.  i've seen a lot of stuff 
in ROOTtalk about how to get the covariance matrix, so i can do that to 
calculate the error myself from the covariance matrix, but it seems like 
there must be something like

    ffgr->GetError(x), or ffgr->EvalError(x), or whatever

to get the error as calculated from the covariance matrix so i don't 
have to do all the work myself.  any ideas?

thanks,

jason



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