DDE-1/Clustering.md

@@ -80,8 +80,13 @@ If you want to learn more about the model, you can follow the details from the [
 
 ### Concept of similarity in machine learning
 
+In both classification and clustering models, we talked about "similarities" between instances, as well as the "distances". It is important to take a breath at this point and think about what they mean in our mathematical framework of the models. 
+
 ...
 
+You can also check:
+[17 types of similarity and dissimilarity measures](https://towardsdatascience.com/17-types-of-similarity-and-dissimilarity-measures-used-in-data-science-3eb914d2681)
+
 ## Additional materials
 
 * [Encode Categorical Variables](https://machinelearningmastery.com/how-to-prepare-categorical-data-for-deep-learning-in-python/)