diff --git a/owlps-positioner/src/posutil.cc b/owlps-positioner/src/posutil.cc
index 41c371e..02b5ff4 100644
--- a/owlps-positioner/src/posutil.cc
+++ b/owlps-positioner/src/posutil.cc
@@ -73,11 +73,19 @@ void PosUtil::complete_with_dummy_measurements(
  * Both lists must have the same size and contain the same keys:
  * you should call complete_with_dummy_measurements() before
  * compute_ss_square_distance().
+ *
+ * The distance between the two lists is computed by averaging the
+ * square distances between the elements of both lists, two by two.
+ * Therefore, it is not really an averaged square distance; you cannot
+ * divide it to obtain an euclidean distance. To do that, one would
+ * have to create a ss_distance() function that average the distances
+ * between the elements of the lists instead of the square distance.
  */
 float PosUtil::ss_square_distance(
   unordered_map<string, Measurement> &measurements1,
   unordered_map<string, Measurement> &measurements2)
 {
+  assert(! measurements1.empty()) ;
   assert(measurements1.size() == measurements2.size()) ;
 
   float distance = 0 ;
@@ -88,11 +96,10 @@ float PosUtil::ss_square_distance(
       unordered_map<string, Measurement>::const_iterator i2 =
         measurements2.find(i1->first) ;
       assert(i2 != measurements2.end()) ;
-      distance += i1->second.ss_square_distance(
-                    i2->second) ;
+      distance += i1->second.ss_square_distance(i2->second) ;
     }
 
-  return distance ;
+  return distance / measurements1.size() ;
 }