Explain what you would like to see improved and how.

It is possible to compute the chi2 between a fitted function TF1 and a histogram TH1 using two different methods:

You usually perform a fit with TH1::Fit first, which automatically computes the chi2 in TF1. You can then get the chi2 with either TF1::GetChisquare, or TH1.Chisquare (which triggers a manual computation). The behavior of the two approaches are basically the same except that the chi2 computed from TH1::Fit also accepts the option I which uses the integral of function in the bin instead of the default bin center value. This option is not available in TH1.Chisquare.

It could be true from a statistics stand point that you don't really need the integral because the fitted function value is suppose to reflect the correct count in that bin? But it would nevertheless be good to have that option available in both ways.

For the fix we probably just need some way to propagate the "I" option in ROOT::Fit::Chisquare to set the proper value of ROOT::Fit::DataOptions.fIntegral

Reproducer:

from array import array

import ROOT

bin_edge = [
    0., 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5,
    0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1.,
]
bin_content = [
    0, 0, 0, 0, 0, 1, 3, 16, 17, 14, 18, 18, 7, 5, 1, 0, 0, 0, 0, 0
]


h = ROOT.TH1D(
    "h_gauss", "",
    len(bin_edge) - 1,
    array("d", bin_edge),
)

for i, val in enumerate(bin_content):
    h.SetBinContent(i + 1, val)
h.Sumw2(ROOT.kFALSE)

f = ROOT.TF1("f_gauss", "gaus", 0.0, 1.0)

h.Fit(f, f"Q")
print(f"Neyman chi2 (useIntegral = False) from TF1::GetChisquare = {f.GetChisquare()}")
print(f"Neyman chi2 (useIntegral = False) from TH1::Chisquare = {h.Chisquare(f, '')}")

# option I for integral over bins instead of bin center
h.Fit(f, f"QI")
print(f"Neyman chi2 (useIntegral = True) from TF1::GetChisquare = {f.GetChisquare()}")
# not supported
print(f"Neyman chi2 (useIntegral = True) from TH1::Chisquare = {h.Chisquare(f, 'I')}")

ROOT version

6.38.00

Installation method

conda

Operating system

Linux

Additional context

No response