Questions about using sumw2 while making ratioplot

Newbie
1

Dear experts,

I am trying to make a ratio plot using two histograms and I saw an example here: ROOT: tutorials/hist/ratioplot1.C File Reference
There is a line calling h1->Sumw2();
I am confused about why do we want to call sumw2 for h1 but not for h2 given the h2 will be scaled afterwards?

Is there a reason for doing that or it’s a typo?

Thanks,
Ang

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2

See the notes on TH1::Scale.

3

Thank you for your reply! So that means calling Scale for h2 automatically calls Sumw2 for h2 so we don’t need to call it explicitly. Will things go wrong if I call Sumw2 multiple times or call Sumw2 together with Scale?

Thanks,
Ang

4

Hi @Ang-Li-95; I think you might get a warning “Sum of squares of weights structure already created”.

Cheers,
J.

5

if (! MyHisto->GetSumw2N()) MyHisto->Sumw2(kTRUE);

6

That is helpful! Thanks!