Hi Rooters,
I was trying to use the error-option in Project3D and I stumbled across
something that I don't understand:
I use a 3D histogram, that has errors defined for every bin (Sumw2). The
errors for the 3d histogram-bins are therefore just the sqrt of the
associated entry. Now the calculation of the error for the projection is
fairly straightforward and the result is that the error for one bin should
just be the sqrt of the total entries projected into that bin (=sqrt(sum
of the squares of the errors of the 3Dhistogram bins) ).
However using the "e" option in Project3D (errors are computed) I get a
result that is way too big (in my humble opinion).
For example:
{
TH3F *test=new TH3F("test","test",2,0.5.,2.5,2,0.5,2.5,2,0.5,2.5);
for(Int_t i=1;i<3;i++){
for(Int_t j=1;j<3;j++){
for (Int_t k=1;k<3;k++) {
test->Fill(i,j,k,10*i);
}
}
}
test->Sumw2();
test->Project3D("xe")->Draw();
cout<<"1st bin content="<<test_xe->GetBinContent(1)<<endl;
cout<<"1st bin error="<<test_xe->GetBinError(1)<<endl;
}
the output is:
1st bin content=40
1st bin error=20.9762
The projection for 2 dimensional histograms,on the other hand works in the
way I expect
{
TH2F *test=new TH2F("test","test",2,0.5.,2.5,2,0.5,2.5);
for(Int_t i=1;i<3;i++){
for(Int_t j=1;j<3;j++) {
test->Fill(i,j,10*i);
}
}
test->Sumw2();
test->ProjectionX("test_px",1,2,"E")->Draw();
cout<<"1st bin content="<<test_px->GetBinContent(1)<<endl;
cout<<"1st bin error="<<test_px->GetBinError(1)<<endl;
}
1st bin content=20
1st bin error=4.47214
I actually looked into the source code and the calculation seems to
correspond to what I expect.
Could someone please explain me what I am doing wrong?
Thanks
Angela
Btw I am using root version 2.23-12 for LINUX
This archive was generated by hypermail 2b29 : Tue Jan 02 2001 - 11:50:21 MET