diff --git a/owlps-positioner/src/measurement.cc b/owlps-positioner/src/measurement.cc
index 0907537..4cf1f86 100644
--- a/owlps-positioner/src/measurement.cc
+++ b/owlps-positioner/src/measurement.cc
@@ -117,6 +117,7 @@ void Measurement::clear()
   average_mw = 0 ;
   variance_mw = 0 ;
   variance_mw_m2 = 0 ;
+  variance_size = 0 ;
   ap = NULL ;
 }
 
@@ -186,6 +187,7 @@ void Measurement::recalculate_average()
   average_mw = 0 ;
   variance_mw = 0 ;
   variance_mw_m2 = 0 ;
+  variance_size = 0 ;
 
   for (map<pkt_id_t, ss_t>::const_iterator
        i = ss_list.begin() ; i != ss_list.end() ; ++i)
@@ -195,18 +197,21 @@ void Measurement::recalculate_average()
 
 void Measurement::update_average(ss_t ss_dbm)
 {
+  ++variance_size ;
+  assert(variance_size <= ss_list.size()) ;
+
   // Convert the new SS in mW:
   float ss_mw = pow(10, static_cast<float>(ss_dbm) / 10.0) ;
 
   // Update the average:
   float delta = ss_mw - average_mw ;
-  average_mw += delta / ss_list.size() ;
+  average_mw += delta / variance_size ;
   average_dbm = 10.0 * log10(average_mw) ;
 
   // Update the variance:
   variance_mw_m2 += delta * (ss_mw - average_mw) ;
-  if (ss_list.size() > 1)
-    variance_mw = variance_mw_m2 / (ss_list.size() - 1) ;
+  if (variance_size > 1)
+    variance_mw = variance_mw_m2 / (variance_size - 1) ;
 }
 
 
@@ -225,6 +230,7 @@ Measurement& Measurement::operator=(const Measurement &m)
   average_mw = m.average_mw ;
   variance_mw = m.variance_mw ;
   variance_mw_m2 = m.variance_mw_m2 ;
+  variance_size = m.variance_size ;
 
   return *this ;
 }
diff --git a/owlps-positioner/src/measurement.hh b/owlps-positioner/src/measurement.hh
index 9e77671..59bbfd7 100644
--- a/owlps-positioner/src/measurement.hh
+++ b/owlps-positioner/src/measurement.hh
@@ -33,8 +33,10 @@ protected:
   float variance_mw ;
 
 private:
-  /// Intermediate variable used to compute the variance
+  /// Intermediate variable used to compute the variance (mW)
   float variance_mw_m2 ;
+  /// Number of elements taken into account in the variance computation
+  unsigned int variance_size ;
 
 
 protected:
@@ -54,12 +56,14 @@ protected:
 public:
   Measurement(const AccessPoint *_ap = NULL):
     ap(const_cast<AccessPoint*>(_ap)), average_dbm(0),
-    average_mw(0), variance_mw(0), variance_mw_m2(0) {}
+    average_mw(0), variance_mw(0), variance_mw_m2(0),
+    variance_size(0) {}
 
   Measurement(const Measurement &source):
     ap(source.ap), ss_list(source.ss_list),
     average_dbm(source.average_dbm), average_mw(source.average_mw),
-    variance_mw(source.variance_mw), variance_mw_m2(0) {}
+    variance_mw(source.variance_mw), variance_mw_m2(0),
+    variance_size(0) {}
 
   ~Measurement(void) ;
 
@@ -73,7 +77,7 @@ public:
   float get_average_dbm() const ;
   /// Returns the variance of the SS list
   float get_variance_mw() const ;
-  /// Returns the standard deviation of the SS list
+  /// Returns the standard deviation of the SS list (mW)
   float get_std_deviation_mw() const ;
   /// Returns the number of SS in the SS list
   int get_nb_ss() const ;