Note that in case of pre-defined functions some default initial values and limits are set. “ B”Use this option when you want to fix one or more parameters and the fitting function is a predefined one, like polN, expo, landau, gaus. “ +” Add this new fitted function to the list of fitted functions (by default, the previous function is deleted and only the last one is kept) By default the fitted function is drawn unless the option “ N” above is specified. “ N” Do not store the graphics function, do not draw “ R” Use the range specified in the function range “ M” Improve fit results, by using the IMPROVE algorithm of TMinuit. “ E” Perform better errors estimation using the Minos technique “ S” The result of the fit is returned in the TFitResultPtr. “ V” Verbose mode (default is between Q and V) The expected error is instead estimated from the the square-root of the bin function value. “ P” Use Pearson chi-square method, using expected errors instead of the observed one given by TH1::GetBinError (default case). To be used when the histogram has been filled with weights different than 1. To be used when the histogram represents counts “ L” Use log likelihood method (default is chi-square method). “ I” Use integral of function in bin instead of value at bin center “ WW” Set all weights to 1 including empty bins ignore error bars “ W” Set all weights to 1 for non empty bins ignore error bars *option: The second parameter is the fitting option. See the next paragraph for the list of pre-defined functions. This name may be one of ROOT pre-defined function names or a user-defined function. TFitResultPtr Fit(TF1 *function, Option_t *option, Option_t *goption, Axis_t xxmin, Axis_t xxmax)įunction a pointer to the fitted function (the fit model) object. Here is the signatures of TH1::Fit and an explanation of the parameters: To fit a histogram programmatically, you can use the TH1::Fit method. The Fit method is implemented in ROOT 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. The method TH1::Fit is more powerful and is used in scripts and programs. The histogram needs to be drawn in a pad before the Fit Panel is invoked.
The Fit Panel, which is limited, is best for prototyping.
To fit a histogram you can use the Fit Panel on a visible histogram via the context menu, or you can use the TH1::Fit method. 7.4 Example of fit: Combining Functions.Chapter: FittingHistograms Chapter: FittingHistograms
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