You are now asking a different question. When you use the Fill functions, the following quantities are incremented Stat_t fEntries; //Number of entries 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 These quantities are not incremented when you use functions such as SetBinContent. When you call functions like GetMean, GetRMS, the parameters above are used to compute the mean or rms when there is no selected range (default). If you have a selected range (eg when zooming on one of the axis), the above parameters cannot be used by definition. In this case the functions compute an approximate mean and rms from the actual bin contents. When you use the TH1::Add function, the above quantities are directly used. The resulting mean or rms will use the full precision information accumulated in these variables. This does not make sense in case of a multiplication/division. In this case, the functions recompute the above parameters from the result of the operation in each bin. Concerning the computation of the errors, assuming histograms h1 and h2, if h1 or h2 have errors stored (TH1::Sumw2 has been called), the resulting errors will be correct. If not, the resulting error bars will be computed as the sqrt of the bin contents. See comments in the functions. Rene Brun Alexander Cherlin wrote: > > Dear Rene! > > > Hi Alexander, > > > > With Add, statistics are computed using the statistical quantities > > in both histograms. In this case, the resulting stats preserve the > > original full accuracy. > > What does it mean "using the statistical quantities > in both histograms"? Is that a weighted mean value? What happens if I > change the content of a bin/bins using SetContent? What is the bin error > in that case if I don't set a new one? > > Thank you, > Alexander. > > > In case of a multiplication/division, statistics are recomputed from > > the bin contents. > > > > Rene Brun > > > > > > Alexander Cherlin wrote: > > > > > > Dear rooters! > > > If I create a new histogram using Add (or Multiply), what show Mean and > > > RMS values in statistics of the new histogram? > > > > > > Thanks in advance, > > > Alexander. > > ************************************************************************ > * Alexander Cherlin | Home: * > * Department of Particle Physics | Pinsker str., 50/15 * > * Weizmann Institute of Science | Rishon le-Zion 75910 * > * Rehovot 76100 ISRAEL | ISRAEL * > * PHONE : 972 8 9343372 | PHONE: 972 3 9560344 * > * FAX : 972 8 9466253 | CELLULAR * > * | PHONE: 972 54 629228 * > * | FAX: 972 15154 629228 * > *----------------------------------------------------------------------* > * E-mail: cherlin@wicc.weizmann.ac.il * > * Alexander.Cherlin@cern.ch * > * URL : http://www.weizmann.ac.il/home/cherlin * > ************************************************************************
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