ROOT   Reference Guide
ROOT Fit Panel

Classes forming the user interface of the Fit Panel in ROOT.

## The Fit Panel

To display the Fit Panel right click on a histogram to pop up the context menu, and then select the menu entry Fit Panel.

By design, this user interface is planned to contain two tabs: "General" and "Minimization". Currently, the "General" tab provides user interface elements for setting the fit function, fit method and different fit, draw, print options. The "Minimization tab" provides the option to set the Minimizer to use in the fit and its specific options.

The fit panel is a modeless dialog, i.e. when opened, it does not prevent users from interacting with other windows. Its first prototype is a singleton application. When the Fit Panel is activated, users can select an object for fitting in the usual way, i.e. by left-mouse click on it. If the selected object is suitable for fitting, the fit panel is connected with this object and users can perform fits by setting different parameters and options.

### Function Choice and Settings

‘Predefined' combo box - contains a list of predefined functions in ROOT. You have a choice of several polynomials, a Gaussian, a Landau, and an Exponential function. The default one is Gaussian.

‘Operation' radio button group defines the selected operational mode between functions:

Nop - no operation (default);

Conv - convolution (will be implemented in the future).

Users can enter the function expression into the text entry field below the ‘Predefined' combo box. The entered string is checked after the Enter key was pressed and an error message shows up, if the function string is not accepted.

‘*Set Parameters*' button opens a dialog for parameters settings, which will be explained later.

### Fitter Settings

‘Method' combo box currently provides only two fit model choices: Chi-square and Binned Likelihood. The default one is Chi-square. The Binned Likelihood is recommended for bins with low statistics.

‘Linear Fit' check button sets the use of Linear fitter when is selected. Otherwise the minimization is done by Minuit, i.e. fit option "F" is applied. The Linear fitter can be selected only for functions linear in parameters (for example - polN).

‘Robust' number entry sets the robust value when fitting graphs.

‘No Chi-square' check button switch On/Off the fit option "C" - do not calculate Chi-square (for Linear fitter).

‘Integral' check button switch On/Off the option "I" - use integral of function instead of value in bin center.

‘Best Errors' sets On/Off the option "E" - better errors estimation by using Minos technique.

‘All weights = 1' sets On/Off the option "W"- all weights set to 1 excluding empty bins; error bars ignored.

‘Empty bins, weights=1' sets On/Off the option "WW" - all weights equal to 1 including empty bins; error bars ignored.

‘Use range' sets On/Off the option "R" - fit only data within the specified function range. Sliders settings are used if this option is set to On. Users can change the function range values by pressing the left mouse button near to the left/right slider edges. It is possible to change both values simultaneously by pressing the left mouse button near to the slider center and moving it to a new position.

‘Improve fit results' sets On/Off the option "M"- after minimum is found, search for a new one.

‘Add to list' sets On/Off the option "+"- add function to the list without deleting the previous one. When fitting a histogram, the function is attached to the histogram's list of functions. By default, the previously fitted function is deleted and replaced with the most recent one, so the list only contains one function. Setting this option to On will add the newly fitted function to the existing list of functions for the histogram. Note that the fitted functions are saved with the histogram when it is written to a ROOT file. By default, the function is drawn on the pad displaying the histogram.

### Draw Options

‘SAME' sets On/Off function drawing on the same pad. When a fit is executed, the image of the function is drawn on the current pad.

‘No drawing' sets On/Off the option "0"- do not draw the fit results.

‘Do not store/draw' sets On/Off option "N"- do not store the function and do not draw it.

The advance option button is enabled only after having performed the fit and provides additional drawing options that can be used after having done the fit. These new drawing tools, which can be selected by the "Advanced Drawing Tool" panel that pops up when clicking the "Advanced" button, are:

• Contour: to plot the confidence contour of two chosen parameters. One can select the number of points to draw the contour (more points might require more time to compute it), the parameters and the desired confidence level .
• Scan : to plot a scan of the minimization function (likelihood or chi-squared) around the minimum as function of the chosen parameter.
• Conf Interval : to plot the confidence interval of the fitted function as a filled coloured band around its central value. One can select the desired confidence level for the band to be plotted.

### Print Options

This set of options specifies the amount of feedback printed on the root command line after performed fits.

‘Verbose' - prints fit results after each iteration.

‘Quiet' - no fit information is printed.

‘Default' - between Verbose and Quiet.

### Command Buttons

Fit button - performs a fit taking different option settings via the Fit Panel interface.

Reset - sets the GUI elements and related fit settings to the default ones.

Close - closes the Fit panel window.

### Minimization Options

With this tab one can select specific options for minimization. These include

• The minimizer library ( Minuit, Minuit2, Fumili, GSL, Genetics )
• The method (algorithm) for minimization. For example for Minuit one can choose between (Migrad, Simplex or Scan)
• Error definition
• Minimization tolerance
• Number of iterations/function calls
• Print Level: (Default, Verbose or Quiet).