Hi Tom, Till, Where do you see the bug ????? Computing the mean value/rms of an histogram by using the bin center is a very crude approximation. In the case of Root: - During the Histogram Fill process, several statistical quantities are incremented (see TH1 members : Stat_t fTsumw; //Total Sum of weights Stat_t fTsumw2; //Total Sum of squares of weights Stat_t fTsumwx; //Total Sum of weight*X Stat_t fTsumwx2; //Total Sum of weight*X*X and the equivalent fot TH2 and TH3. There are obviously cases where the above parameters are not available, eg when you SetBinContent yourself. In this case, Root uses the poor man approach of computing these quantities using the bin center only. Rene Brun Till Neunhoeffer wrote: > > Hi all, > > we have done the following: > > 1) Create some histogram a and fill it > 2) create an identical histogram b > 3) for each bin in a, fill a->GetBinContent in the corresponding bin of b > > For all we know, these histograms should be identical. > > Yet their means and RMS differ. > > This seems to derive from the fact that when filling the histogram a, its > mean gets calculated with the actual x value at which we fill and not with > the center of the corresponding bin. > > Is this a bug or a feature? > > Thanks > Tom and Till
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