diff --git a/owlps-positioner/src/measurement.cc b/owlps-positioner/src/measurement.cc
index 1048584..8f882a6 100644
--- a/owlps-positioner/src/measurement.cc
+++ b/owlps-positioner/src/measurement.cc
@@ -127,6 +127,9 @@ void Measurement::clear()
 
 float Measurement::similarity(const Measurement &source) const
 {
+  assert(! ss_list.empty()) ;
+  assert(! source.ss_list.empty()) ;
+
   string algorithm(
     Configuration::string_value("positioning.ss-similarity")) ;
 
@@ -134,24 +137,27 @@ float Measurement::similarity(const Measurement &source) const
     return ss_square_distance(source) ;
 
   if (algorithm == "interval")
-    {
-      float std_dev = get_std_deviation() ;
-
-      // Count the number of SS of source that are within the interval
-      unsigned int nb_values = 0 ;
-      for (map<pkt_id_t, ss_t>::const_iterator ss =
-             source.ss_list.begin() ; ss != source.ss_list.end() ; ++ss)
-        if (PosUtil::is_in_interval(average_dbm, std_dev, ss->second))
-          ++nb_values ;
-
-      return 1 / nb_values ;
-    }
+    return 1 / nb_in_interval(source, get_std_deviation()) ;
 
   throw bad_configuration(
     "Bad SS similarity algorithm name \""+ algorithm +"\"!") ;
 }
 
 
+unsigned int Measurement::
+nb_in_interval(const Measurement &source, float bound) const
+{
+  unsigned int nb_values = 0 ;
+
+  for (map<pkt_id_t, ss_t>::const_iterator ss =
+         source.ss_list.begin() ; ss != source.ss_list.end() ; ++ss)
+    if (PosUtil::is_in_interval(average_dbm, bound, ss->second))
+      ++nb_values ;
+
+  return nb_values ;
+}
+
+
 void Measurement::recalculate_average()
 {
   average_dbm = 0 ;
diff --git a/owlps-positioner/src/measurement.hh b/owlps-positioner/src/measurement.hh
index b6228f2..8f0ceea 100644
--- a/owlps-positioner/src/measurement.hh
+++ b/owlps-positioner/src/measurement.hh
@@ -44,6 +44,10 @@ protected:
   void recalculate_average(void) ;
   /// Update the average and variance with a new SS
   void update_average(ss_t ss_dbm) ;
+  /// \brief Counts the number of SS of source that are within the
+  /// interval [average_dbm-bound;average_dbm+bound]
+  unsigned int nb_in_interval(
+    const Measurement &source, float bound) const ;
   //@}