diff --git a/ChangeLog b/ChangeLog
index 91d71410..e7e02cc6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -53,6 +53,9 @@ FIXED 2012-09-23 fixed wrong chisq output in musrview if expected chisq is
                  present.
 FIXED 2012-05-30 fixed RRF  bug in single histo plotting.
 FIXED 2012-05-18 fixed wrong forward/backward tag for ROOT-PPC (MUSR-215) 
+CHANGED 2013-11-12 changed normalization of log max likelihood according to S.
+Backer and R.D. Cousins NIM 221, 437 (1984), in order to have a
+"goodness-of-fit" criteria.
 CHANGED 2012-12-11 if multiple SAVE are present in the COMMAND block, append
                    MINUIT2.OUTPUT file. Added docu for PRINT_LEVEL (MUSR-244).
 CHANGED 2012-11-19 replaced hard coded gyromagnetic ratio of the muon in the
diff --git a/src/classes/PRunMuMinus.cpp b/src/classes/PRunMuMinus.cpp
index 4e4f95fa..e61d5d55 100644
--- a/src/classes/PRunMuMinus.cpp
+++ b/src/classes/PRunMuMinus.cpp
@@ -261,6 +261,11 @@ Double_t PRunMuMinus::CalcMaxLikelihood(const std::vector<Double_t>& par)
 
     data = fData.GetValue()->at(i);
 
+    if (theo <= 0.0) {
+      cerr << ">> PRunMuMinus::CalcMaxLikelihood: **WARNING** NEGATIVE theory!!" << endl;
+      continue;
+    }
+
     if (data > 1.0e-9) {
       mllh += (theo-data) + data*log(data/theo);
     } else {
@@ -268,7 +273,7 @@ Double_t PRunMuMinus::CalcMaxLikelihood(const std::vector<Double_t>& par)
     }
   }
 
-  return mllh;
+  return 2.0*mllh;
 }
 
 //--------------------------------------------------------------------------
diff --git a/src/classes/PRunSingleHisto.cpp b/src/classes/PRunSingleHisto.cpp
index 7dc81d05..b18092f3 100644
--- a/src/classes/PRunSingleHisto.cpp
+++ b/src/classes/PRunSingleHisto.cpp
@@ -376,14 +376,19 @@ Double_t PRunSingleHisto::CalcMaxLikelihood(const std::vector<Double_t>& par)
     chunk = 10;
   #pragma omp parallel for default(shared) private(i,time,theo,data) schedule(dynamic,chunk) reduction(-:mllh)
   #endif
-  for (i=startTimeBin; i < endTimeBin; ++i) {
+  for (i=startTimeBin; i<endTimeBin; ++i) {
     time = fData.GetDataTimeStart() + (Double_t)i*fData.GetDataTimeStep();
     // calculate theory for the given parameter set
-    theo = N0*TMath::Exp(-time/tau)*(1+fTheory->Func(time, par, fFuncValues))+bkg;
+    theo = N0*TMath::Exp(-time/tau)*(1.0+fTheory->Func(time, par, fFuncValues))+bkg;
     theo *= normalizer;
 
     data = normalizer*fData.GetValue()->at(i);
 
+    if (theo <= 0.0) {
+      cerr << ">> PRunSingleHisto::CalcMaxLikelihood: **WARNING** NEGATIVE theory!!" << endl;
+      continue;
+    }
+
     if (data > 1.0e-9) {
       mllh += (theo-data) + data*log(data/theo);
     } else {
@@ -391,7 +396,7 @@ Double_t PRunSingleHisto::CalcMaxLikelihood(const std::vector<Double_t>& par)
     }
   }
 
-  return mllh;
+  return 2.0*mllh;
 }
 
 //--------------------------------------------------------------------------