Feldman Causing and large signal uncertainty

Roofit and RooStats
1

Hello,

(I use root 6.03/04)
I’m learning roostats and I was trying to do something simple: evaluate the importance of uncertainties on limits. I took rs101_limitexample.C, set observed = background on l 86, and then started increasing uncertainties. If I increase the background uncertainty on l 72 things go south pretty quickly:
at 10% (as in rs101) I get

Profile upper limit on s = 28.5301
FC upper limit on s = 35.4
MCMC upper limit on s = 33.3135

at 20% the Feldman-Cousins test starts going south:
Profile upper limit on s = 44.309
FC upper limit on s = 90.6
MCMC upper limit on s = 52.0216

at 30% FC hits its scan limit
Profile upper limit on s = 62.4388
FC upper limit on s = 119.4
MCMC upper limit on s = 72.4625

Is there something special I need to do to make FC “behave” in the presence of larger uncertainties? No change I’ve tried in number of samples etc. has made things better.

Thx

Gustaaf

2

Hi,

Thank you for pointing to this problem. Actually it is recommended for running Feldman-Cousins to use the FrequentistCalculator and the HypoTestInverter class as it is done for limit settings using CLs.

I see also the way of doing the model is not 100% right. The model does not contain the global observables needed to correctly include the systematics in FC.
n
I attached a new corrected version of the macro. I addition the macro saves in a file the workspace.
After running the rs101 you can run also the roostats/StandardHypoTestInvDemo.C macro to use the recommended way of doing FeldmanCousins. For example do at the root prompt

.x rs101_limitexample.C 
.L  $ROOTSYS/tutorials/roostats/StandardHypoTestInvDemo.C
root [2] StandardHypoTestInvDemo("rs101_ws.root","w","ModelConfig","","exampleData",0,2,0,10,0,150)

Cheers

Lorenzo
rs101_limitexample.C (9.29 KB)

3

Thanks, this helps a lot.