Hi Colin, SetBinError sets the sum of squares of weights in the data structure. In case of a TProfile, the computation of errors uses this information, but also the number of entries in a bin. In general, it does not make much sense to call this function for a TProfile See the various options for errors in TProfile. May be what you want to do is the following: TH1D *proj = profile->ProjectionX(); proj->setBinError(... loop Rene Brun Rene Brun Colin Bernet wrote: > > Hi all, > > I don't understand what is happening when using SetBinError on a TProfile > . au->fHxbja1d is a TProfile, and I was expecting SetBinError to set the > error for the bin, but it does something different : > > root [12] au->fHxbja1d->GetBinError(1) > (const Stat_t)6.10376002362651021e-04 > root [13] au->fHxbja1d->SetBinError(1,0.1) > root [14] au->fHxbja1d->GetBinError(1) > (const Stat_t)1.21475547561875668e-04 > > Could somebody explain ? > > I'm using /afs/cern.ch/sw/root/v3.05.07/rh73_gcc2952/root/ > > thank you ! > Colin
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