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  1. ROOT
  2. ROOT-6664

Problems fitting sub-ranges with RooChebychev



    • Bug
    • Resolution: Fixed
    • High
    • 5.34/24
    • None
    • RooFit/RooStats
    • None
    • Mac Os X 10.9.4 (X86-64)
      ROOT 5.34/20
      Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)
      Target: x86_64-apple-darwin13.3.0
      Thread model: posix



      I am performing a fit of a blinded dataset using a RooChebychev.
      I have already posted this issue on the ROOT forum in June, but I didn't get any reply. Now we are a little stuck with our analysis because we need to be able to perform this kind of fit.

      I did lots of checks try to debug the problem, but I didn't manage to find any good solution.

      In attachment you can find a macro that shows the problem. The macro is divided into "TEST1" and "TEST2".
      – TEST1 generates a dataset using a 1st order Chebychev in the mass region [4700,5900] and a "blinded" dataset reducing the full dataset to the two sub-regions [4700,5100] and [5500,4900]. Then, it performs two fits:
      1) fit the "full" dataset and plot the results (lines 75-79 )
      2) fit the "blinded" dataset and plots the results (lines 86-90).

      – TEST2, similar to TEST1, but with a different PDF (1st order Chebychev + Gaussian)

      Now, if I do this test using a RooExponential, the fit is OK on both full and blinded dataset (you can try that commenting RooChebychev and de-commenting RooExponential), whereas if I do the test using RooChebychev, when I do the fit on the two sub-ranges, ROOT crashes and exits due to the assert() present in the RooChebychev.cxx (lines 141-142). In attachment you can find the bt obtained running lldb on my laptop.

      I added some prints to RooChebychev.cxx


      Now, looking at RooChebychev.h, if I understood correctly, "_x" should correspond to the variable "mass" (in my case) and so xminfull = _x.min() should always be equal to 4700, and xmaxfull = _x.max() should always be equal to 5900.
      When I try to do the fit in the two sub-ranges I got this print (before ROOT crashes)

      _x.min(rangeName) = 4700
      _x.max(rangeName) = 5900
      xminfull = 4700
      xmaxfull = 5100

      As you can see, it seems that the ranges are swapped, and so the assert() fails and exits ROOT. "rangeName" should be "SB1" or "SB2" (defined in lines 54-55).

      I have tried to investigate the problem in more detail, but I din't find any proper solution. Maybe there is some problem in dealing with the ranges in the RooChebychev.cxx (maybe some problem in RooProxy?).
      Looking at RooAbsOptTestStatistic.cxx, in lines 286-365 RooFit make adjustments for the fit ranges, I saw that, commenting line 311, ROOT does not crash, the fit with the Chebychev only seems to be OK (but not the normalisation, that is wrong), whereas the fit Chebychev+Gaussian converges but the chi2 is huge (so the fitted curve does not fit the data and the normalisation is wrong).

      I hope these tests can help in figure out where the problem is. Let me know if I can help with some other tests and if you need more information.

      Thanks a lot in advance!




        1. Debug_report.txt
          5 kB
        2. FitUnblindedDataset.C
          5 kB
        3. RooChebychev.cxx
          7 kB
        4. RooChebychev.h
          2 kB



            verkerke Wouter Verkerke
            calpigia Cristiano Alpigiani
            7 Vote for this issue
            6 Start watching this issue


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