# How to select an optimal number of clusters in my example?

I want to group my dataset using clustering technique. I apply k-means and used Dunn index and Silhouette Coefficient for the validation (selection of the best number of clusters). Now I want to know what should be the optimal cluster number based on the Dunn index. For your reference i am uploading the DI and SC plot ("cluster size" is the number of clusters).

The point is that if I keep on increasing the cluster no's the DI value is getting higher after 7. The minimum value in figure is 5. So can't we take 5 as the min no of cluster possible.

Please suggest what should be the cluster size i need to consider for this plot.

• I reckon to go with 7 clusters? en.wikipedia.org/wiki/Dunn_index#Explanation – Jan Sila Sep 6 '16 at 9:48
• cluster size What's that? Are you speaking about the number of clusters? – ttnphns Sep 6 '16 at 10:09
• Yes. I want to find the optimal no of clusters. – Chandra Prakash Sep 6 '16 at 11:05
• There exist plenty, plenty internal clustering criterions (validation criterions). Each one has its "preferences" or "biases". Dunn's index and Silhouette index are just two of the many. By the way, both these two exist in original as well as in modified versions. The two are quite different conceptually and don't have to be concordant most of the time. In your example, Dunn suggests the 7 clusters and Silhouette - the 2 clusters. If you want a forced negotiation solution you might take the 3-cluster solution as not bad. – ttnphns Sep 6 '16 at 11:26

• But I strongly recommend to always 1. visualize the clusters. 2. carefully study the resulting clusters, don't just assume they are good. So do I. Mind, however, that eye is just one among "clustering criterions" with its own prejudices, so visualization, even if easily possible, isn't the ultimate judge. Some reasonable good and interpretable cluster solutions may be perceived by eye as inconvincing. – ttnphns Sep 6 '16 at 21:29