Sum of weight * weight * X * X in TH1

ROOT
1

Hello everyone,
TH1 histograms keep track of the total sum of weights, of the total sum of squares of weights, of the total sum of weight * X, and of the total sum of weight * X * X. These can be accessed via TH1::GetStats.

Is the total sum of weight * weight * X * X also computed? It is useful when one wants to evaluate the variance of weight*X.

Thanks a lot,
Claudio

2

Hi,
The histogram does not store the sum of weight * weight * X * X. But, why would you need to for ? One is interested in the weighted variance of X not the variance of weight * X.
If this is what you need fill an histogram with (weight * x) as new variable X

Cheers

Lorenzo

3

Thank you Lorenzo for your reply!

I understand this may be a niche application, but when computing a weighted average it can be useful to evaluate the numerator, sum w_i x_i, and the denominator, sum w_i, separately, and to combine them at a later stage. To keep track of uncertainties, this requires the variance of weight * x to be evaluated.

If this is what you need fill an histogram with (weight * x) as new variable X

Indeed, this seems to be the best solution given what ROOT currently does.

Thanks,
Claudio