SetParLimits Ignored in Fit

Hi, I am having trouble fitting a simple TH1D with a Gaussian function. It appears that my SetParLimits calls are not being obeyed. Here is a fragment of my PyROOT code that does this:

# h is a TH2D, h_binedges is a python list of pairs of floats
projs, fit_gaus = {}, {}
for i in range(len(h_binedges)-1):
projs[i] = h.ProjectionY("_%d" % i,h_binedges[i],h_binedges[i+1]-1)
fg_min,fg_max = projs[i].GetXaxis().GetXmin(),projs[i].GetXaxis().GetXmax()
fg_height = projs[i].GetMaximum()
fg_centre = projs[i].GetMean()
fg_width = projs[i].GetRMS()

    fit_gaus[i] = ROOT.TF1("fit_gaus_%d" % i,"gaus",fg_min,fg_max)
    fit_gaus[i].SetParameters(fg_height, fg_centre, fg_width/3.5)

    ftr_p = projs[i].Fit(fit_gaus[i],"SRQ+")
    ftr = ftr_p.Get()
    if int(ftr_p) != 0 or not (0.5*fg_height < ftr.Parameter(0) < 2*fg_height) or not (1e-3 < ftr.Parameter(2) < 2*fg_width) or abs(ftr.Parameter(1) - fg_centre)/ftr.Parameter(2) > 5 :
        refit_func = fit_gaus[i].Clone("refit_%d" % i)
        ftr_p = projs[i].Fit(refit_func,"SRW+")
        ftr = ftr_p.Get()
        fit_gaus[i] = refit_func
    print i, ftr.Parameter(0), ftr.Parameter(1), ftr.Parameter(2)[/code]

After the fit, the printed values of the parameters are often outside of the limits that I set. This happens even on iterations where the “refit” clause was not executed, so I don’t think it’s related to that.

So what could cause my SetParLimits to be ignored here? Any help would be appreciated.


Try with the “B” fit option.

Yes, you should use the “B” option, since you are using a pre-defined function (“gaus”). IN that case the fit internally re-sets the initial parameter values and limits to optimal ones.
The option “B” is used to avoid this and use the values provided by the user

Best Regards


Ah, thanks for pointing that out. I did see the “B” option, but I think I was mis-led by the wording of the description:

[quote]User defined parameter settings are used for predefined functions like “gaus”, “expo”, “poln”, “landau”.
Use this option when you want to fix one or more parameters for these functions.[/quote]

Can we change the second sentence to be less specifically about “fixing” parameters? Something like:


See the TF1::SetParLimits method description if you like misleading information. It simply claims that you must use “B” whenever you want to “set limits” (“predefined functions” are not even mentioned): [quote]The specified limits will be used in a fit operation when the option “B” is specified (Bounds).[/quote]