Setting error

ROOT
1

Hi ,

  I have a very simple query about setting error using SetBinError() method.

  I have a histogram filled like h->Fill(x,w); where "w" is the weight for each entry. After filling "h" I decide to apply different scales to each bin and set new content as (scale_i * x) for each bin "i". In this case how I  should get and set the correct error (using SetBinError() method) for each bin in the new histogram?

Thanks in advance,
sushil

2

In the ref guide, there is some info concerning the errors of a scaled histogram …
Have you seen it ?

root.cern.ch/doc/master/classTH … ae0860bfbd

3

Hi,

Thanks for your prompt reply. Yes I have seen this earlier but not sure I fully understand. I am aware that h->Fill() will take care error properly if declared h->Sumw2(), but here i am making a new histogram by ->SetBinContent() method.

I am applying a different scale to each bin to my “h” histogram to get a new histogram like this:
h->Fill(x, w); // then stored in a root file
// later I do this
h_new->SetBinContent(i, h->GetBinContent(i) * scale[i]); // i is for each bin scale is different for each i
h_new->SetBinError(???);

My query is what ?? should be in this case.

Thanks again,
Regards,
sushil

4

Hi,

If you are scaling yourself the histogram content using TH1::SetBinContent you should also scale the error using
TH1::SetBinError.

Cheers

Lorenzo

5

so is this the correct way?
h_new->SetBinError(i, (h->GetBinError(i) * scale[i]) );

( because I understand h->GetBinError(i) is sum of square of weights used while filling h)

sushil

6

okay, I got the answer and it is the correct way. Thanks you all for your reply.

regards,sushil