diff --git a/volia/measures.py b/volia/measures.py
index 6d5d721..71f0908 100644
--- a/volia/measures.py
+++ b/volia/measures.py
@@ -152,6 +152,15 @@ def entropy_score(y_truth, y_hat):
 
 
 def purity_score(y_truth, y_hat):
+    '''
+    Return three values in a dictionary:
+        - purity_class_score: the purity score of the class (asp)
+        - purity_cluster_score: the purity score of the cluster (acp)
+        - K: the overall evaluation criterion (sqrt(asp * acp))
+
+    This function is based on the following article: 
+    Unknown-multiple speaker clustering using HMM, J. Ajmera, H. Bourlard, I. Lapidot, I. McCowan
+    '''
 
     def divide_line(a, divider):
         '''