The Davies-Bouldin index

The Davies-Bouldin index (BDI) is the ratio of intra-cluster distance to inter-cluster distance. A lower DBI value means better clustering results. This can be calculated as follows:

Here, the following applies:

• n: The number of clusters
• c_i: The centroid of cluster i
• σ_i: The intra-cluster distance or average distance of all cluster items from the centroid c_i
• d(c_i, c_j): The inter-cluster distance between two cluster centroids c_i and c_j