Hi Steffen, By default, errors bars are computed as sqrt(bin content). If you want correct error bars to be computed and stored, you have to call dndy1->Sumw2(); before filling the histogram in T.Draw(.. Rene Brun On Fri, 3 Dec 1999, Steffen A. Bass wrote: > > Hello folks! > > I have a simple greenhorn problem with the normalization of a 1D > histogram: > > I have a Tree T containing a variable fY, the distribution of which I want > to plot in a histogram, normalized per event. > > So, first I create the histogram (30 bins from -2 to 2): > > dndy1 = new TH1F("dndy","dN/dy of pi0",30,-2.0,2.0); > > Then I use T.Draw to fill the histo and plot it with error-bars: > > T.Draw("fY>>dndy","1/(5000*4/30)*(fId==111)","e1"); > > I have normalized my histogram so that the integral of the distribution > gives me the number of selected entries (particles) per event): > "1/(5000*4/30)" = 1/number_of_events_in_tree * 1/bin_width > > However, the error-bars are not getting renormalized (i.e. rescaled) > properly (or at all) - the result is far too large error bars for the > distribution. > > What am I doing wrong and how to I get the proper error-bars for my > normalized histogram? > > Would it not be more convenient to at least have the division by the > bin-width automatically done in the histogramming, i.e. in the fill > method? > > Thanks a lot for the help! > > Steffen > > PS: I have tried to fill out the registration form, but somehow your > computer does not respond when I click on the "submit registration" > button... > > -------------------------------------------------------------------------- > Steffen A. Bass email: bass@nscl.msu.edu > Cyclotron Laboratory http://www.nscl.msu.edu/~bass > Michigan State University office: (517) 333-6434 > East Lansing, MI 48823 FAX: (517) 353-5967 > -------------------------------------------------------------------------- > >
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