Using Poisson CI bin errors

Hello experts,

I am trying to use Poisson confidence interval bin errors, instead of the usual square root. The histogram in question already seems to have the EBinErrorOpt set to 1 (kPoisson), but the errors it gives seem to be the default ones. That is, just the square root of the bin count. Running these simple commands, gives:

root [1] data_obs->GetBinErrorOption()
(TH1::EBinErrorOpt) (TH1::EBinErrorOpt::kPoisson) : (unsigned int) 1
root [2] data_obs->GetBinContent(1,7)
(double) 3.0000000
root [3] data_obs->GetBinErrorUp(1,7)
(double) 1.7320508
root [4] data_obs->GetBinContent(1,8)
(double) 0.0000000
root [5] data_obs->GetBinErrorUp(1,8)
(double) 0.0000000

For example, if Poisson CI errors are used, the last line should be around 1.8. What seems to be the problem?
Attached is the .root file with the histogram.

ROOT Version: 6.20/02
Platform: Ubuntu 18.04.5
Compiler: linuxx8664gcc
sample.root (4.0 KB)


Is your histogram weighted ? In that case if the sum of weights^2 is not equals to the bin content then the Poisson error cannot be computed


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