Hi,
Here’s a simple test case that illustrates a problem fitting in 5.27 that works fine in 5.22:
#! /usr/bin/env python
import ROOT
from array import array
if __name__ == "__main__":
ROOT.gROOT.SetStyle('Plain')
ROOT.gROOT.SetBatch()
yVal = array ('d', [0.95, 1, 1.1])
xVal = array ('d', [-1, 0, 1])
canvas = ROOT.TCanvas ()
graph = ROOT.TGraph ( len (xVal), xVal, yVal)
graph.SetMarkerStyle (20)
graph.SetMarkerColor (ROOT.kRed)
graph.SetMarkerSize (1.4)
graph.SetMaximum (1.3)
graph.SetMinimum (0.7)
fun = ROOT.TF1("fun", 'pol2')
fun.FixParameter(0, 1)
graph.Fit (fun)
graph.Draw ('AP')
canvas.Print ('fit.png')Running the code in 5.27b:
cplager@cmslpc08> fitSTC.py
****************************************
Minimizer is Linear
Chi2 = 2
NDf = 0
p0 = 1 (fixed)
p1 = 0.075 +/- 0.707107
p2 = 1.025 +/- 0.707107 
Running the code in 5.22:
cplager@cmslpc11> ./fitSTC.py
****************************************
Minimizer is Linear
Chi2 = 6.16298e-32
NDf = 0
Par_0 = 1 (fixed)
Par_1 = 0.075 +/- 0.707107
Par_2 = 0.025 +/- 0.707107
Note that the third parameter Par_2 is 0.025 in the latter case and 1.025 in the former.
Cheers,
Charles