Rf 1 0 9_Chi 2Residpull¶

'BASIC FUNCTIONALITY' RooFit tutorial macro #109 Calculating chi^2 from histograms and curves in ROOT.RooPlots, making histogram of residual and pull distributions

Author: Wouter Verkerke (C version)
This notebook tutorial was automatically generated with ROOTBOOK-izer from the macro found in the ROOT repository on Monday, January 17, 2022 at 09:52 AM.

In [ ]:
from __future__ import print_function
import ROOT

Set up model¶

Create observables

In [ ]:
x = ROOT.RooRealVar("x", "x", -10, 10)

Create Gaussian

In [ ]:
sigma = ROOT.RooRealVar("sigma", "sigma", 3, 0.1, 10)
mean = ROOT.RooRealVar("mean", "mean", 0, -10, 10)
gauss = ROOT.RooGaussian("gauss", "gauss", x, mean, sigma)

Generate a sample of 1000 events with sigma=3

In [ ]:
data = gauss.generate({x}, 10000)

Change sigma to 3.15

In [ ]:
sigma.setVal(3.15)

Plot data and slightly distorted model¶

Overlay projection of gauss with sigma=3.15 on data with sigma=3.0

In [ ]:
frame1 = x.frame(ROOT.RooFit.Title("Data with distorted Gaussian pdf"), ROOT.RooFit.Bins(40))
data.plotOn(frame1, ROOT.RooFit.DataError(ROOT.RooAbsData.SumW2))
gauss.plotOn(frame1)

Calculate chi^2¶

Show the chi^2 of the curve w.r.t. the histogram If multiple curves or datasets live in the frame you can specify the name of the relevant curve and/or dataset in chiSquare()

In [ ]:
print("chi^2 = ", frame1.chiSquare())

Show residual and pull dists¶

Construct a histogram with the residuals of the data w.r.t. the curve

In [ ]:
hresid = frame1.residHist()

Construct a histogram with the pulls of the data w.r.t the curve

In [ ]:
hpull = frame1.pullHist()

Create a frame to draw the residual distribution and add the distribution to the frame

In [ ]:
frame2 = x.frame(ROOT.RooFit.Title("Residual Distribution"))

Create a frame to draw the pull distribution and add the distribution to the frame

In [ ]:
frame3 = x.frame(ROOT.RooFit.Title("Pull Distribution"))

c = ROOT.TCanvas("rf109_chi2residpull", "rf109_chi2residpull", 900, 300)
c.Divide(3)
c.cd(1)
frame1.GetYaxis().SetTitleOffset(1.6)
frame1.Draw()
c.cd(2)
frame2.GetYaxis().SetTitleOffset(1.6)
frame2.Draw()
c.cd(3)