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RooUnfold: ROOT Unfolding Framework

RooUnfold is a framework for unfolding (AKA "deconvolution" or "unsmearing"). It currently implements five methods: RooUnfold was written by Tim Adye, Richard Claridge, Kerstin Tackmann, and Fergus Wilson. It can be used from the ROOT prompt, or linked against the ROOT libraries. It requires ROOT 4 or later.

Please let me know if you use this software. This will further encourage me to continue working on it, and I will let you know about any future updates.

See this overview of RooUnfold or the references below for more information. To cite the RooUnfold package in a publication, you can refer to this web page and/or the paper:
Tim Adye, in Proceedings of the PHYSTAT 2011 Workshop on Statistical Issues Related to Discovery Claims in Search Experiments and Unfolding, CERN, Geneva, Switzerland, 17–20 January 2011, edited by H.B. Prosper and L. Lyons, CERN–2011–006, pp. 313–318.

Unfolding Method

We use unfolding to remove the known effects of measurement resolutions, systematic biases, and detection efficiency to determine the "true" distribution. We parametrise the measurement effects using a response matrix that maps the (binned) true distribution onto the measured one. For 1-dimensional true and measured distribution bins Tj and Mi, the response matrix element Rij gives the fraction of events from bin Tj that end up measured in bin Mi. The response matrix is usually determined using Monte Carlo simulation (training), with the true values coming from the generator output.

The unfolding procedure reconstructs the true Tj distribution from the measured Mi distribution, taking into account the measurement uncertainties due to statistical fluctuations in the finite measured sample (without these uncertainties, the problem could be solved uniquely by inverting the response matrix). RooUnfold provides several algorithms for solving this problem.

The iterative and SVD unfolding algorithms require a regularisation parameter to prevent the statistical fluctuations being interpreted as structure in the true distribution. It is therefore necessary to optimise this parameter for the number of bins and sample size, using Monte Carlo samples of the same size as the data. These samples can also be used to measure the effectiveness of the unfolding and hence provide estimates of the systematic errors that result from the procedure (testing).

Note that for this last step (in particular), it is important to use Monte Carlo samples with truth distributions that are statistically and systematically independent of the sample used in training (such samples would anyway be used in a systematics analysis, eg. using a different generator, or reweighting variations within the a-priori uncertainties in the truth distribution). After all, if the Monte Carlo were a perfect model of the data, we could use the Monte Carlo truth information directly and dispense with unfolding altogether!

The bin-by-bin method assumes no migration of events between bins (eg. resolution is much smaller than the bin size and no systematic shifts). This is of course trivial to implement without resorting to the RooUnfold machinery, but is included in the package to allow simple comparison with the other methods.

Using RooUnfold

To use RooUnfold, we must first supply the response matrix object RooUnfoldResponse. It can be constructed like this:

RooUnfoldResponse response (nbins, x_lo, x_hi);
or, if different truth and measured binning is required,
RooUnfoldResponse response (nbins_measured, x_lo_measured, x_hi_measured,
                            nbins_true,     x_lo_true,     x_hi_true);
or, if different binning is required,
RooUnfoldResponse response (hist_measured, hist_truth);

In that last case, hist_measured and hist_truth are used to specify the dimensions of the distributions (the histogram contents are not used here), eg. for 2D or 3D distributions or non-uniform binning.

This RooUnfoldResponse object is often most easily filled by looping over the training sample and calling response.Fill(x_measured,x_true) or, for events that were not measured due to detection inefficiency, response.Miss(x_true)

if (measurement_ok)
  response.Fill (x_measured, x_true);
  response.Miss (x_true);
Alternatively, the response matrix can be constructed from a pre-existing TH2D 2-dimensional histogram (with truth and measured distribution TH1D histograms for normalisation).

This response object can be passed directly to the unfolding object, or written to a ROOT file for use at a later stage (search for examples/RooUnfoldTest.cxx's stage parameter for an example of how to do this).

To do the unfolding (either to try different regularisation parameters, for testing, or for real data), create a RooUnfold object and pass it the test / measured distribution (as a histogram) and the response object.

RooUnfoldBayes    unfold (&response, hist_measured, iterations);
RooUnfoldSvd      unfold (&response, hist_measured, kterm);
RooUnfoldBinByBin unfold (&response, hist_measured);
hist_measured is a pointer to a TH1D (or TH2D for the 2D case) histogram of the measured distribution (it should have the same binning as the response matrix). The classes RooUnfoldBayes, RooUnfoldSvd, and RooUnfoldBinByBin all inherit from RooUnfold and implement the different algorithms. The integer iterations (for RooUnfoldBayes) or kterm (RooUnfoldSvd) is the regularisation parameter. (Note that RooUnfoldSvd's kterm parameter is also known as tau in the code. That usage is incompatible with the literature, so we adopt k here.)

The reconstructed truth distribution (with errors) can be obtained with the Hreco() method.

TH1D* hist_reco= (TH1D*) unfold.Hreco();
The result can also be obtained as as a TVectorD with full TMatrixD covariance matrix.

Multi-dimensional distributions can also be unfolded, though this does not work for the SVD method, and the interface is rather clumsy (we hope to improve this).

See the class documentation for details of the RooUnfold and RooUnfoldResponse public methods.

A very simple example of RooUnfold's use is given in examples/RooUnfoldExample.cxx. More complete tests, using different toy MC distributions, are in examples/RooUnfoldTest.cxx and examples/RooUnfoldTest2D.cxx.

Choice of Regularisation Parameter

The regularisation parameter determines the relative weight placed on the data, compared to the training sample truth. Both RooUnfoldBayes and RooUnfoldSvd take integer regularisation parameters, with small values favouring the MC truth and larger values favouring the data.

RooUnfoldBayes Regularisation

For RooUnfoldBayes, the regularisation parameter specifies the number of iterations, starting with the training sample truth (iterations=0). You should choose a small integer greater than 0 (we use 4 in the examples). Since only a few iterations are needed, a reasonable performance can usually be obtained without fine-tuning the parameter.

The optimal regularisation parameter can be selected by finding the largest value up to which the errors remain reasonable (ie. do not become much larger than previous values). This will give the smallest systematic errors (reconstructed distribution least biased by the training truth), without too-large statistical errors. Since the statistical errors grow quite rapidly beyond this point, but the systematic bias changes quite slowly below it, it can be prudent to reduce the regularisation parameter a little below this optimal point.

RooUnfoldSvd Regularisation

For RooUnfoldSvd, the unfolding is something like a Fourier expansion in "result to be obtained" vs "MC truth input". Low frequencies are assumed to be systematic differences between the training MC and the data, which should be retained in the output. High frequencies are assumed to arise from statistical fluctuations in data and unfortunately get numerically enhanced without proper regularization. Choosing the regularization parameter, k (kterm), effectively determines up to which frequencies the terms in the expansion are kept. (Actually, this is not quite true, we don't use a hard cut-off but a smooth one.)

The correct choice of k is of particular importance for the SVD method. A too-small value will bias the unfolding result towards the MC truth input, a too-large value will give a result that is dominated by unphysically enhanced statistical fluctuations. This needs to be tuned for any given distribution, number of bins, and approximate sample size — with k between 2 and the number of bins. (Using k=1 means you get only the training truth input as result without any corrections. You basically regularise away any differences, and only keep the leading term which is, by construction, the MC truth input.)

Höcker and Kartvelishvili's paper (section 7) describes how to choose the optimum value for k.

Building the Library

Make sure that ROOT is set up correctly: the $ROOTSYS environment variable should point to the top-level ROOT directory, $ROOTSYS/bin should be in your $PATH, and $ROOTSYS/lib should be in your library path ($LD_LIBRARY_PATH on most Unix systems). In recent versions of ROOT (from 5.18), this can be most easily achieved using ROOT's thisroot.(c)sh setup script. Eg. to use the CERN AFS version 5.28/00a on Scientific Linux 4/5 (x32) from a Bourne-type shell:
shell> . /afs/
For further details, consult the ROOT "Getting Started" documentation, or your local system administrator.

Download RooUnfold-1.1.1.tar.gz (or other versions here) and unpack

shell> tar zxf RooUnfold-1.1.1.tar.gz
shell> cd RooUnfold-1.1.1
Use GNU make. Just type
shell> make
to build the RooUnfold shared library.

Loading RooUnfold

The RooUnfold library can be used from the ROOT prompt, from a CINT script run from ROOT, from code compiled in ROOT (ACLiC), or linked into a stand-alone program.

If using ROOT (CINT or ACLiC), the library can be loaded automatically when a RooUnfold class is first used. This only works if your current directory is the RooUnfold top-level directory (containing or that directory has been added to your dynamic path. Otherwise, you can load the library with gSystem->Load(), eg.

root [0] gSystem->Load("/home/RooUnfold-1.1.1/libRooUnfold");
root [1] RooUnfoldResponse response(10,-1,1);

To use ACLiC, you also need to add the RooUnfold headers to the include path. This can be done with .include src from the ROOT command prompt, eg.

root [0] .include src
root [1] .x MyCode.cxx+

To build stand-alone, you need to specify the RooUnfold headers in the src subdirectory and specify the -lRooUnfold library. Alternatively, you can use make (with RooUnfold's GNUmakefile) to compile and link your own code, eg. make MyProgram will compile MyProgram.cxx and link with RooUnfold.

Running the Examples

examples/RooUnfoldExample.cxx makes a simple test of RooUnfold.
shell> root
root [0] .x examples/RooUnfoldExample.cxx
More involved tests, allowing different toy MC PDFs to be used for training and testing, can be found in examples/RooUnfoldTest.cxx (which uses test class RooUnfoldTestHarness). To run RooUnfoldTest from within ROOT:
root [1] .x examples/RooUnfoldTest.cxx

The example programs can also be run from the shell command line.

shell> make bin
shell> ./RooUnfoldTest
and the output appears in

You can specify parameters for RooUnfoldTest (either as an argument to the routine, or as parameters to the program), eg.

root [2] RooUnfoldTest("method=2 ftestx=3")
shell> ./RooUnfoldTest method=2 ftestx=3
Use RooUnfoldTest -h or RooUnfoldTest("-h")to list all the parameters and their defaults.

method specifies the unfolding algorithm to use:

0  no unfolding (output copied from measured input)
1  Bayes
2  SVD
3  bin-by-bin
4  TUnfold
5  matrix inversion
ftrainx and ftestx specify training and test PDFs:
0  flat distribution
1  top-hat distribution over mean ± 3 x width
2  Gaussian
3  Double-sided exponential decay
4  Breit-Wigner
5  Double Breit-Wigner, peaking at mean-width and mean+width
6  exponential
7  Gaussian resonance on an exponential background
The centre and width scale of these signal PDFs can be specified with the mtrainx and wtrainx (and mtestx and wtestx) parameters respectively. A flat background of fraction btrainx (and btestx) is added. Detector effects are modelled with a variable shift (xbias in the centre, dropping to 0 near the edges), a smearing of xsmear bins, as well as a variable efficiency (between effxlo at xlo and effxhi at xhi).

For 2D and 3D examples look at RooUnfoldTest2D and RooUnfoldTest3D.

Testing without RooFit

The test programs, examples/RooUnfoldTest.cxx, examples/RooUnfoldTest2D.cxx, and examples/RooUnfoldTest3D.cxx use RooFit to generate the toy distributions. (RooFit is not required to use the RooUnfold classes from another program, eg. examples/RooUnfoldExample.cxx). Hand-coded alternatives are provided if ROOT was not build with RooFit enabled (eg. --enable-roofit not specified). This version generates peaked signal events over their full range, so may have a fewer events within the range than requested.

To disable the use of RooFit, #define NOROOFIT before loading RooUnfoldTest*.cxx

root [0] #define NOROOFIT 1
root [1] .x examples/RooUnfoldTest.cxx
For the stand-alone case, use
shell> make bin NOROOFIT=1
to build (this is the default if RooFit is not available).



The principal RooUnfold developer and contact is Tim Adye (RAL,

The TUnfold interface, matrix inversion, and bin-by-bin algorithms as well as the error and parameter analysis frameworks were written by Richard Claridge (RAL).

The SVD algorithm was written by Kerstin Tackmann (LBNL) for the unfolding of the hadronic mass spectrum in B→Xu.

An initial implementation of the iterative Bayesian algorithm was written by Fergus Wilson (RAL).

Further Development

RooUnfold is now (July 2010) being developed as part of the Unfolding Framework Project.

The latest development version of RooUnfold is maintained in the project's Subversion code repository, and can be viewed here (WebSVN, or viewvc). It can be checked out using:

shell> svn co RooUnfold


To cite the RooUnfold package in a publication, you can refer to this web page and/or the paper:
Tim Adye, in Proceedings of the PHYSTAT 2011 Workshop on Statistical Issues Related to Discovery Claims in Search Experiments and Unfolding, CERN, Geneva, Switzerland, 17–20 January 2011, edited by H.B. Prosper and L. Lyons, CERN–2011–006, pp. 313–318.


  1. T. Adye, RooUnfold: unfolding framework and algorithms, presentations to the Oxford and RAL ATLAS Groups, May 2008. This includes a brief introduction to unfolding but the description of RooUnfold is now dated (see below for a more recent description).
  2. G. Cowan, A Survey of Unfolding Methods for Particle Physics, Proc. Advanced Statistical Techniques in Particle Physics, Durham (2002).
  3. G. Cowan, Statistical Data Analysis, Oxford University Press (1998), Chapter 11: Unfolding
  4. R. Barlow, SLUO Lectures on Numerical Methods in HEP (2000), Lecture 9: Unfolding


  1. T. Adye, Unfolding algorithms and tests using RooUnfold, write-up of a presentatation at the PHYSTAT 2011 workshop on unfolding and deconvolution, CERN (January 2011).
    Previously presented at the Alliance Workshop on Unfolding and Data Correction, DESY (May 2010).
  2. A. Höcker and V. Kartvelishvili, SVD Approach to Data Unfolding, NIM A 372 (1996) 469.
  3. BaBar Collaboration, B. Aubert et al., Study of b→ulν Decays on the Recoil of Fully Reconstructed B Mesons and Determination of |Vub|, hep-ex/0408068, contribution to the 32nd International Conference on High Energy Physics (ICHEP'04, Beijing 2004).
  4. G. D'Agostini, A multidimensional unfolding method based on Bayes theorem, NIM A 362 (1995) 487.
    1. V. Blobel, Unfolding Methods in High Energy Physics Experiments, DESY 84-118 (1984) and Proc. 8th CERN School of Computing (Aiguablava, Spain, 1984), CERN 85-09.
    2. V. Blobel, An unfolding method for high energy physics experiments, Proc. Advanced Statistical Techniques in Particle Physics, Durham (2002).
  5. Alliance Workshop on Unfolding and Data Correction, DESY (May 2010).
  6. PHYSTAT 2011 workshop on unfolding and deconvolution, CERN (January 2011).


See the History file for details of changes to the code. A summary of changes to the code and this web page are:–

11 October 2012: Add formal citation to PHYSTAT 2011 paper.

17 November 2011: Add Loading RooUnfold section to detail how to load the RooUnfold libraries.

10 October 2011: update to version 1.1.1:

30 September 2011: update to version 1.1.0:

6 May 2011: reference RooUnfold write-up from PHYSTAT 2011.

9 February 2011: reference the PHYSTAT 2011 workshop on unfolding and deconvolution.

14 January 2011: update to version 1.0.3:

13 September 2010: update to version 1.0.2:

30 July 2010: use in ROOT setup example. Update RooUnfoldTest help files.

3 June 2010: reference the Alliance Workshop on Unfolding and Data Correction.

20 May 2010: update to version 0.2.2:

19 May 2010: update to version 0.2.1:

22nd January 2010: update to version 0.2.0:

14th October 2009: update to version 0.1.9:

13th August 2008: add brief instructions for setting up ROOT.

13th May 2008: updated RooUnfold slides for a talk I gave today. Updated SPIRES URL.

23rd January 2008: update to version 0.1.5:

2nd August 2007: update to version 0.1.4 with these changes to the SVD algorithm from Kerstin Tackmann:

12th July 2007: mention TH1D and TH2D classes explicitly. RooUnfold only supports histograms of doubles, not eg. TH1F.

17th April 2007: first public version.

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