Fitting histograms
Fitting is the method for modeling the expected distribution of events in a physics data analysis. ROOT offers various options to perform the fitting of the data:
 Fit Panel: After a histogram is drawn, the Fit Panel GUI is best used for prototyping the fit.
 Fit() method: You can fit histograms and graphs programmatically with the
Fit()
method.  Minimization packages: ROOT provides several minimization packages like Minuit2 and FUMILI.
 RooFit: The RooFit library is a toolkit for modeling the expected distribution of events in a physics analysis.
Using the Fit Panel
After you have drawn a histogram (see → Drawing a histograms), you can use the Fit Panel for fitting the data. The Fit Panel is best suited for prototyping
The following section describes how to use the Fit Panel using an example.
Given is a histogram following a Gaussian distribution.

Rightclick on the object and then click
FitPanel
.
You also can selectTools
and then clickFit Panel
.
Figure: FitPanel in the context menu.
The Fit Panel is displayed.
Figure: Fit Panel.
In the Fit Function
section you can select the function that should be used for fitting.
The following types of functions are listed here:

Predefined functions that will depend on the dimensionality of the data.

Functions present in
gDirectory
. These functions were already created by the user through a ROOT macro. 
Previously used functions. Functions that fitted the current data previously, if the data is able to store such functions.
Select a fitting function.
 Click
Set Parameters...
to set the parameters of the selected function.
The Set Parameters of...
dialog window is displayed.
Figure: Set Parameters of… dialog window.

Set the parameters for the fit function.

In the
General
tab, select the general options for fitting.
This includes the method that will be used, as well as what fit options will be used with it and the draw options. You can also constrain the range of the function used for the fitting. 
In the
Minimization
tab, select the minimization algorithm for fitting. 
Click
Fit
.
Figure: A fitted histogram.
Using the Fit() method
The Fit()
method is implemented for:

the histogram classes TH1

the sparse histogram classes THnSparse

the graph classes TGraph , TGraph2D and TMultiGraph (for fitting a collection of graphs with the same function)
Using TH1::Fit()
 Use the TH1::Fit() method to fit a histogram programmatically.
By default, the fitted function object is added to the histogram and is drawn in the current pad.
The signature is:
function
: Pointer to the fitted function (the fit model) object.
option
: The fitting option, with the following options:
W
: Sets all weights to 1 for non empty bins; ignore error bars.WW
: Sets all weights to 1 including empty bins; ignore error bars.I
: Uses integral of function in bin instead of value at bin center.L
: Uses a log likelihood method (default is chisquare method). To be used when the histogram represents counts.WL
: Weighted log likelihood method. To be used when the histogram has been filled with weights different than 1.P
: Uses Pearson chisquare method. Uses expected errors instead of the observed one given by TH1::GetBinError() (default case). The expected error is instead estimated from the squareroot of the bin function value.Q
: Quiet mode (minimum printing).V
: Verbose mode (default is betweenQ
andV
).S
: The result of the fit is returned in the TFitResultPtr .E
: Performs better errors estimation using the Minos technique.M
: Improves fit results, by using the IMPROVE algorithm of TMinuit .R
: Uses the range specified in the function range.N
: Does not store the graphics function, does not draw.0
: Does not plot the result of the fit. By default the fitted function is drawn unless the optionN
is specified.+
: Adds this new fitted function to the list of fitted functions (by default, the previous function is deleted and only the last one is kept).B
: Use this option when you want to fix one or more parameters and the fitting function is a predefined one, likepolN
,expo
,landau
,gaus
.
Note that in case of predefined functions, some default initial values and limits are set.C
: In case of linear fitting, do no calculate the chisquare (saves time).F
: If fitting a linear function (e.g.,polN
), switch to use the default minimizer (e.g., TMinuit ). By default,polN
functions are fitted by the linear fitter.
goption
: The graphics option that is the same as TH1::Draw().
xxmin
, xxmax
: Specifies the range over which to apply the fit.
Using TGraph::Fit()
The signature for fitting TGraph is the same as for the TH1 .
Only the following options only apply for fitting histograms:
L
WL
I
The following options only apply for TGraph::Fit:
EX0
: When fitting a TGraphErrors or a TGraphAsymmErrors , the errors on the coordinates are not used in the fit.ROB
: Use the Robust fitting in case of linear fitting . Computes the LTS regression coefficients (robust (resistant) regression), using the default fraction of good points.ROB=0.x
: As above, but compute the LTS regression coefficients, using 0.x as a fraction of good points.
Using the TF1 function class
In the following section is described how to use the TF1 class that is used for fitting histograms and graphs.
Fitting 1D histograms with predefined functions
 Use the TH1::Fit() method to fit a 1D histogram with a predefined function. The name of the predefined function is the first parameter. For predefined functions, you do not need to set initial values for the parameters.
Example
A histogram object hist
is fit with a Gaussian:
The following predefined functions are available:

gaus
: Gaussian function with three parameters:f(x) = p0*exp(0.5*((xp1)/p2)ˆ2)

expo
: Exponential function with two parameters:f(x) = exp(p0+p1*x)

polN
: Polynomial of degreeN
, whereN
is a number between 0 and 9:f(x) = p0 + p1*x + p2*x2 +...

chebyshevN
: Chebyshev polynomial of degreeN
, whereN
is a number between 0 and 9:f(x) = p0 + p1*x + p2*(2*x21) +...

landau
: Landau function with mean and sigma. This function has been adapted from theCERNLIB
routineG110 denlan
(see → TMath::Landau). 
gausn
: Normalized form of the Gaussian function with three parametersf(x) = p0*exp(0.5*((xp1)/p2)ˆ2)/(p2*sqrt(2PI))
Fitting 1D histograms with userdefined functions
First you create a TF1
object, then use the name of the TF1
fitting function in the Fit() method.
You can create the TF1
fitting function as follows:

from an existing expressions defined in TFormula (with and without parameters),

by defining your own function.
Creating a TF1 fitting function with a TFormula expression
Example
The TF1
constructor is used with the formula sin(x)/x
.
You can also use a TF1 object in the constructor of another TF1 object.
Example
The TF1
constructor is used with the formula x*sin(x)
and two parameters.
The parameter index is enclosed in square brackets.
Use SetParameter()
to set the initial values.
Use SetParameters()
to set multiple parameters at once.
Creating a user TF1 fitting function
You can define your own function and then pass the function pointer to the TF1 constructor. Your function for a TF1 constructor must have the following signature:
Double_t *x
: Pointer to the variable array. This array must be a 1D array with v[0] = x
in case of a 1D histogram, v[0] =x
, v[1] = y
for a 2D histogram, etc.
Double_t *par
: Pointer to the parameter array. par
contains the current values of parameters when it is called by the FCN()
function.
Example
An 1D histogram is fit with a userdefined function.
See also the fitexample.C
tutorial.
Now the fitf
function is used to fit the histogram.
Accessing the fitted function parameters and results
 Use the TH1::GetFunction() method to access the fitted function parameters.
Examples
Configuring the fit
The following configuration actions are available when fitting a histogram or graph using the Fit()
method:
Fixing and setting parameter bounds
For predefined functions like poln
, exp
, gaus
, and landau
, the parameter initial values are set automatically.
For not predefined functions, the fit parameters must be initialized before invoking the Fit()
method.
 Use the TF1::SetParLimits() method to set the bounds for one parameter.
When the lower and upper limits are equal, the parameter is fixed.
Example
The parameter is fixed 4 at 10.
 Use the TF1::FixParameter() method to fix a parameter to 0.
Example
You do not need to set the limits for all parameters.
Example
There is function with 6 parameters. Then there is a setup possible like the following: parameters 0 to 2 can vary freely, parameter 3 has boundaries [10, 4] with the initial value 1.5, and parameter 4 is fixed to 0.
Fitting subranges
By default, TH1::Fit() fits the function on the defined histogram range. You can specify the R
option in the second
parameter of TH1::Fit()
to restrict the fit to the range specified in the TF1
constructor.
Example
The fit will be limited to 3 to 3, the range specified in the TF1
constructor:
You can also specify a range in the call to TH1::Fit()
.
See also the ROOT macros $ROOTSYS/tutorials/fit/myfit.C
and multifit.C
for more detailed examples.
Fitting multiple sub ranges
You can find a ROOT macro for fitting multiple sub ranges at $ROOTSYS/tutorials/fit/multifit.C
. It shows how to use several Gaussian functions
with different parameters on separate sub ranges of the same histogram.
Example
Four TF1 objects are created, one for each sub range.
The histogram are filled with bins defined in the array x
.
When fitting simple functions, such as a Gaussian, the initial values of the parameters are automatically computed. In the more complicated case of the sum of 3 Gaussian functions, the initial values of parameters must be set. In this particular case, the initial values are taken from the result of the individual fits.
Result of the fit
You can obtain the following results of a fit:
Associated function
One or more objects (typically a TF1\*
) can be added to the list of functions (fFunctions
) associated to each histogram.
TH1::Fit() adds the fitted function to this list.
Given a histogram h
, you can retrieve the associated function with:
Accessing the fit parameters and results
If the histogram or graph is made persistent, the list of associated functions is also persistent.
Retrieve a pointer to the function with the TH1::GetFunction() method. Then you can retrieve the fit parameters from the function.
Example
With the fit option S
, you can access the full result of the fit including the covariance and correlation matrix.
Associated errors
By default, for each bin, the sum of weights is computed at fill time. You can also call TH1::Sumw2() to force the storage
and computation of the sum of the square of weights per bin. If Sumw2()
has been called, the error per bin is computed
as the sqrt(sum of squares of weights). Otherwise, the error is set equal to the sqrt(bin content).
To return the error for a given bin number, use:
Empty bins are excluded in the fit when using the Chisquare fit method. When fitting an histogram representing
counts (this is with Poisson statistics) it is recommended to use the LogLikelihood method (option L
or WL
), particularly
in case of low statistics.
Fit statistics
You can change the statistics box to display the fit parameters with the TStyle::SetOptFit() method. This parameter has four digits: mode = pcev (default = 0111)
p = 1
: Print probability.c = 1
: Print Chisquare/number of degrees of freedom.e = 1
: Print errors (if e=1, v must be 1).v = 1
: Print name/values of parameters.
Example
To print the fit probability, parameter names/values, and errors, use:
Using ROOT::Fit classes
ROOT::Fit is the namespace for fitting classes (regression analysis). The fitting classes are part of the MathCore library.
The defined classes can be classified in the following groups:
 Fit method classes: Classes describing fit method functions like:
 ROOT::Fit::Chi2FCN: Class for binned fits using the least square methods.
 ROOT::Fit::PoissonLikelihoodFCN: Class for evaluating the log likelihood for binned Poisson likelihood fits.
 ROOT::Fit::LogLikelihoodFCN: Calls for likelihood fits.

Fit data classes: Classes for describing the input data for fitting. These classes are, among others, ROOT::Fit::BinData, for binned data sets (data points containing both coordinates and a corresponding value/weight with optionally an error on the value or the coordinate), and ROOT::Fit::UnBinDatac, for unbinned data sets.
 User fitting classes: Classes for fitting a given data set.
Creating the input data
There are two types of input data:
 Binned data (ROOT::Fit::BinData): They are used for least square (chisquare) fits of histograms or TGraph objects.
 Unbinned data (ROOT::Fit::UnBinData): They are used for fitting vectors of data points, for example from a TTree [TTree].
Using binned data
 Use the ROOT::Fit::BinData class for binned data.
Example
There is histogram, represented as a TH1
type object. Now a ROOT:Fit::BinData
object is created and filled.
By using ROOT::Fit::DataOptions you can specify the data range and some fitting options.
Using unbinned data
 Use the ROOT::Fit::UnBinData class for unbinned data.
For creating unbinned data sets, there are two possibilities:
 Copy the data inside a
ROOT::Fit::UnBinData
object.
Create an emptyROOT::Fit::UnBinData
object, iterate on the data and add the data point one by one. An inputROOT::Fit::DataRange
object is passed in order to copy the data according to the given range.  Use
ROOT::Fit::UnBinData
as a wrapper to an external data storage.
In this case theROOT::Fit::UnBinData
object is created from an iterator or pointers to the data and the data are not copied inside. The data cannot be selected according to a specified range. All the data points will be included in the fit.
ROOT::Fit::UnBinData
supports also weighted data. In addition to the data points (coordinates), which
can be of arbitrary k
dimensions, the class can be constructed from a vector of weights.
Example
Data are taken from a histogram (TH1 object).
Creating a fit model
The model function needs to be expressed as function of some unknown parameters. The fitting will find the best parameter value to describe the observed data.
You can for example use the TF1 class, the parametric function class to describe the model function. But the ROOT::Fit::Fitter class takes as input a more general parametric function object, the abstract interface class ROOT::Math::IParametricFunctionMultiDim. It describes a generic onedimensional or multidimensional function with parameters. This interface extends the abstract ROOT::Math::IBaseFunctionMultiDim class with methods to set/retrieve parameter values and to evaluate the function given the independent vector of values X and vector of parameters P.
Configuring the fit
Use the ROOT::Fit::FitConfig (contained in the ROOT::Fit::ParameterSettings class) class for configuring the fit.
There the following fit configurations:
 Setting the initial values of the parameters.
 Setting the parameter step sizes.
 Setting eventual parameter bounds.
 Setting the minimizer library and the particular algorithm to use.
 Setting different minimization options (print level, tolerance, max iterations, etc. . . ).
 Setting the type of parameter errors to compute (parabolic error, minor errors, renormalize errors using fitted chi2 values).
Example
Setting the lower/upper bounds for the first parameter and a lower bound for the second parameter:
Performing the fit
Depending on the available input data and the selected function for fitting, you can use one of the methods of the ROOT::Fit::Fitter class to perform the fit.
The following predefined fitting methods are available:

Leastsquare fit: Fitter::LeastSquare(const BinData &) or Fitter::Fit(const Bindata &). Both methods should be used when the binned data values follow a Gaussian distribution. These fit methods are implemented using the class ROOT::Fit::Chi2FCN.

Binned likelihood fit: Fitter::LikelihoodFit(const Bindata &). This method should be used when the binned data values follow a Poisson or a multinomial distribution. The Poisson case (extended fit) is the default and in this case the function normalization is also fit to the data. This method is implemented by the ROOT::Fit::PoissonLikelihoodFCN class.

Unbinned likelihood fit: Fitter::LikelihoodFit(const UnBindata &). By default the fit is not extended, this is the normalization is not fitted to the data. This method is implemented using the LogLikelihoodFCN class.

Linear fit: A linear fit can be selected, if the model function is linear in the parameters.
Fit result
The result of the fit is contained in the ROOT::Fit::Result object.
You can print the result of the fit with the FitResult::Print() method.