From e86b424179d0e34a4411b96d64a73fd7592d2672 Mon Sep 17 00:00:00 2001
From: Lorenzo Moneta <Lorenzo.Moneta@cern.ch>
Date: Fri, 20 Jun 2008 15:40:26 +0000
Subject: [PATCH] - implement in the Profile the existing option G to get the
bin error in the case of combination of measurements y with gaussian
error dy. In this case the profile must be filled with a weight = 1./dy**2
and the correct error from the combination (1./sqrt(Sum(1./dy**2) ) ) will be
obtained. This option was requested by Yuri Fisyak.
git-svn-id: http://root.cern.ch/svn/root/trunk@24431 27541ba8-7e3a-0410-8455-c3a389f83636
---
hist/hist/src/TProfile.cxx | 31 +++++++++++++++++++------------
1 file changed, 19 insertions(+), 12 deletions(-)
diff --git a/hist/hist/src/TProfile.cxx b/hist/hist/src/TProfile.cxx
index 8b6cab9c89e..19e8e6290dd 100644
--- a/hist/hist/src/TProfile.cxx
+++ b/hist/hist/src/TProfile.cxx
@@ -210,15 +210,17 @@ void TProfile::BuildOptions(Double_t ymin, Double_t ymax, Option_t *option)
// uncertainty is +-0.5, with the assumption that the probability that Y
// takes any value between Y-0.5 and Y+0.5 is uniform (the same argument
// goes for Y uniformly distributed between Y and Y+1); this would be
-// useful if Y is an ADC measurement, for example. Other, fancier options
+// useful if Y is an ADC measurement, for example.
+// Other, fancier options
// would be possible, at the cost of adding one more parameter to the PROFILE
// command. For example, if all Y variables are distributed according to some
-// known Gaussian of standard deviation Sigma, then:
-// 'G' Errors are Spread/SQRT(N) for Spread.ne.0. ,
-// " " Sigma/SQRT(N) for Spread.eq.0,N.gt.0 ,
-// " " 0. for N.eq.0
+// known Gaussian of standard deviation Sigma (which can be different for each measurement),
+// and the profile has been filled with a weight equal to 1/Sigma**2,
+// then one cam use the following option:
+//
+// 'G' Errors are 1./SQRT(Sum(1/sigma**2))
// For example, this would be useful when all Y's are experimental quantities
-// measured with the same instrument with precision Sigma.
+// measured with different precision Sigma_Y.
//
//
@@ -925,7 +927,8 @@ Double_t TProfile::GetBinError(Int_t bin) const
return 1/TMath::Sqrt(12*sum);
}
else if (fErrorMode == kERRORSPREADG) {
- return eprim/TMath::Sqrt(sum);
+ // it is supposed the values y are gaussian distributed y +/- dy
+ return 1./TMath::Sqrt(sum);
}
else return eprim;
}
@@ -1438,10 +1441,8 @@ TH1D *TProfile::ProjectionX(const char *name, Option_t *option) const
// product of the bin content of the profile and the entries.
// With this option the returned histogram will be equivalent to the one obtained by
// filling directly a TH1D using the 2-nd value as a weight.
-// This option makes sense only for profile filled with all weights =1.
-// When the profile is weighted (filled with weights different than 1) the
-// bin error of the projected histogram (obtained using this option "W") cannot be
-// correctly computed from the information stored in the profile.
+// This makes sense only for profile filled with weights =1. If not, the error of the
+// projected histogram obtained with this option will not be correct.
TString opt = option;
@@ -1487,7 +1488,7 @@ TH1D *TProfile::ProjectionX(const char *name, Option_t *option) const
// if option E projected histogram errors are same as profile
if (computeErrors ) h1->SetBinError(bin , GetBinError(bin) );
// in case of option W bin error is deduced from bin sum of z**2 values of profile
- // this is correct only if the profile is unweighted
+ // this is correct only if the profile was filled with weights = 1.
if (binWeight) h1->SetBinError(bin , TMath::Sqrt(fSumw2.fArray[bin] ) );
}
@@ -1850,6 +1851,12 @@ void TProfile::SetErrorOption(Option_t *option)
// 'i' Errors are Spread/SQRT(N) for Spread.ne.0. ,
// " " 1./SQRT(12.*N) for Spread.eq.0,N.gt.0 ,
// " " 0. for N.eq.0
+// 'g' Errors are 1./SQRT(W) for Spread.ne.0. ,
+// " " 0. for N.eq.0
+// W is the sum of wights of the profile.
+// This option is for measurements y +/ dy and the profile is filled with
+// weights w = 1/dy**2
+//
// See TProfile::BuildOptions for explanation of all options
TString opt = option;
--
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