This example is a generalization of the on/off problem.
This example is a generalization of the on/off problem. It's a common setup for SUSY searches. Imagine that one has two variables "x" and "y" (eg. missing ET and SumET), see figure. The signal region has high values of both of these variables (top right). One can see low values of "x" or "y" acting as side-bands. If we just used "y" as a sideband, we would have the on/off problem.
- In the signal region we observe non events and expect s+b events.
- In the region with low values of "y" (bottom right) we observe noff events and expect tau*b events. Note the significance of tau. In the background only case:
tau ~ <expectation off> / <expectation on>
If tau is known, this model is sufficient, but often tau is not known exactly. So one can use low values of "x" as an additional constraint for tau. Note that this technique critically depends on the notion that the joint distribution for "x" and "y" can be factorized. Generally, these regions have many events, so it the ratio can be measured very precisely there. So we extend the model to describe the left two boxes... denoted with "bar".
- In the upper left we observe nonbar events and expect bbar events
- In the bottom left we observe noffbar events and expect tau bbar events Note again we have:
tau ~ <expectation off bar> / <expectation on bar>
One can further expand the model to account for the systematic associated to assuming the distribution of "x" and "y" factorizes (eg. that tau is the same for off/on and offbar/onbar). This can be done in several ways, but here we introduce an additional parameter rho, which so that one set of models will use tau and the other tau*rho. The choice is arbitrary, but it has consequences on the numerical stability of the algorithms. The "bar" measurements typically have more events (& smaller relative errors). If we choose
<expectation noffbar> = tau * rho * <expectation noonbar>
the product tau*rho will be known very precisely (~1/sqrt(bbar)) and the contour in those parameters will be narrow and have a non-trivial tau~1/rho shape. However, if we choose to put rho on the non/noff measurements (where the product will have an error ~1/sqrt(b))
, the contours will be more amenable to numerical techniques. Thus, here we choose to define:
tau := <expectation off bar> / (<expectation on bar>)
rho := <expectation off> / (<expectation on> * tau)
^ y
|
|---------------------------+
| | |
| nonbar | non |
| bbar | s+b |
| | |
|---------------+-----------|
| | |
| noffbar | noff |
| tau bbar | tau b rho |
| | |
+-----------------------------> x
Left in this way, the problem is under-constrained. However, one may have some auxiliary measurement (usually based on Monte Carlo) to constrain rho. Let us call this auxiliary measurement that gives the nominal value of rho "rhonom". Thus, there is a 'constraint' term in the full model: P(rhonom | rho). In this case, we consider a Gaussian constraint with standard deviation sigma.
In the example, the initial values of the parameters are:
- s = 40
- b = 100
- tau = 5
- bbar = 1000
- rho = 1
(sigma for rho = 20%)
and in the toy dataset:
- non = 139
- noff = 528
- nonbar = 993
- noffbar = 4906
- rhonom = 1.27824
Note, the covariance matrix of the parameters has large off-diagonal terms. Clearly s,b are anti-correlated. Similarly, since noffbar >> nonbar, one would expect bbar,tau to be anti-correlated.
This can be seen below.
GLOBAL b bbar rho s tau
b 0.96820 1.000 0.191 -0.942 -0.762 -0.209
bbar 0.91191 0.191 1.000 0.000 -0.146 -0.912
rho 0.96348 -0.942 0.000 1.000 0.718 -0.000
s 0.76250 -0.762 -0.146 0.718 1.000 0.160
tau 0.92084 -0.209 -0.912 -0.000 0.160 1.000
Similarly, since tau*rho appears as a product, we expect rho,tau to be anti-correlated. When the error on rho is significantly larger than 1/sqrt(bbar), tau is essentially known and the correlation is minimal (tau mainly cares about bbar, and rho about b,s). In the alternate parametrization (bbar* tau * rho) the correlation coefficient for rho,tau is large (and negative).
The code below uses best-practices for RooFit & RooStats as of June 2010.
It proceeds as follows:
- create a workspace to hold the model
- use workspace factory to quickly create the terms of the model
- use workspace factory to define total model (a prod pdf)
- create a RooStats ModelConfig to specify observables, parameters of interest
- add to the ModelConfig a prior on the parameters for Bayesian techniques note, the pdf it is factorized for parameters of interest & nuisance params
- visualize the model
- write the workspace to a file
- use several of RooStats IntervalCalculators & compare results
Processing /mnt/build/workspace/root-makedoc-v608/rootspi/rdoc/src/v6-08-00-patches/tutorials/roostats/FourBinInstructional.C...
void FourBinInstructional(bool doBayesian=false, bool doFeldmanCousins=false, bool doMCMC=false){
wspace->
factory(
"Poisson::on(non[0,1000], sum::splusb(s[40,0,100],b[100,0,300]))");
wspace->
factory(
"Poisson::off(noff[0,5000], prod::taub(b,tau[5,3,7],rho[1,0,2]))");
wspace->
factory(
"Poisson::onbar(nonbar[0,10000], bbar[1000,500,2000])");
wspace->
factory(
"Poisson::offbar(noffbar[0,1000000], prod::lambdaoffbar(bbar, tau))");
wspace->
factory(
"Gaussian::mcCons(rhonom[1.,0,2], rho, sigma[.2])");
wspace->
factory(
"PROD::model(on,off,onbar,offbar,mcCons)");
wspace->
defineSet(
"obs",
"non,noff,nonbar,noffbar,rhonom");
wspace->
factory(
"Uniform::prior_poi({s})");
wspace->
factory(
"Uniform::prior_nuis({b,bbar,tau, rho})");
wspace->
factory(
"PROD::prior(prior_poi,prior_nuis)");
plc.SetConfidenceLevel(0.95);
fc.SetConfidenceLevel(0.95);
fc.FluctuateNumDataEntries(false);
fc.UseAdaptiveSampling(true);
fc.SetNBins(40);
if(doFeldmanCousins){
}
bc.SetConfidenceLevel(0.95);
if(doBayesian && wspace->
set(
"poi")->
getSize() == 1) {
bInt = bc.GetInterval();
} else{
cout << "Bayesian Calc. only supports on parameter of interest" << endl;
}
mc.SetConfidenceLevel(0.95);
mc.SetProposalFunction(*pf);
mc.SetNumBurnInSteps(500);
mc.SetNumIters(50000);
mc.SetLeftSideTailFraction(0.5);
if(doMCMC)
mcInt = mc.GetInterval();
if(!c1)
if(doBayesian && doMCMC){
}
else if (doBayesian || doMCMC){
}
if(doBayesian && wspace->
set(
"poi")->
getSize() == 1) {
bc.SetScanOfPosterior(20);
RooPlot* bplot = bc.GetPosteriorPlot();
}
if(doMCMC){
if(doBayesian && wspace->
set(
"poi")->
getSize() == 1)
else
mcPlot.Draw();
}
cout << "Profile Likelihood interval on s = [" <<
if(doBayesian && wspace->
set(
"poi")->
getSize() == 1) {
cout << "Bayesian interval on s = [" <<
}
if(doFeldmanCousins){
cout << "Feldman Cousins interval on s = [" <<
}
if(doMCMC){
cout << "MCMC interval on s = [" <<
}
}
- Authors
- authors: Kyle Cranmer, Tanja Rommerskirchen
Definition in file FourBinInstructional.C.