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?

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.

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.

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