TH2F ProjectionX to TH1D, how error transfer is done

[Chen123]

hello, I have two Dalitz plot of h1, h2(TH2F) and use h1/h2 two get the efficiency.
h1->SumW2();
h1->Divide(h2);

Then I get the projection in x/y direction:
TH1D *h1x = h1->ProjectionX();
TH1D *h1y = h1->ProjectionY();

The error of each bin in h1 is normal, but the error of some bins in h1x or h1y is very large.
I do not know why, may be the error in the projection plot is wrong?

[jonas]

Hi @Chen123, can you please share these histograms so that I can try to reproduce the problem? If you can’t share them let me know, then I’ll try to reproduce it with some toy histograms.

[Chen123]

Thank you. I have already solved this problem. The value and error of each bin in h1x and h1y is the sum in each direction of h1. I have some mistake before which lead to the difference.

[moneta]

Hi,

Instead of summing efficiency which does not make much sense, you might want to take the average of them. In this case you should use h1->ProfileX() instead of ProjectionX

Lorenzo