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On the right side you see an explanation and more details regarding the clustering quality measure.

Davies Bouldin Index (R)

The Davies Bouldin Index relates the average distance of elements of each cluster to their respective centroids to the distance of the centroids of the two clusters. \[ DB = \frac{1}{n} \cdot \sum_{c_i \in C} \underset{c_i \neq c_j}{\text{max}} \left( \frac{\sigma_i + \sigma_j}{d(\bar{c_i},\bar{c_j})} \right) \]