Is TMath::Probe different from PROB of cernlib?

Hello

I compared TMath::Prob and PROB of cernlib

TMath::Prob(88.4, 34) => 1.60006e-07
PROB(88.4, 34) => 9.94584e-07

why they are different?

Hi,

the implementation of TMath::Prob is known to be not very accurate.
You should use for the time being the function in MatHCore:

ROOT::Math::chisquared_cdf_c(88.4,34)

which will give you a more correct result.

In the next ROOT release we will also re-implement TMath::Prob using this new function.

Best Regards

Lorenzo