Another common method is called average linkage (or Unweighted Pair Group Method with Arithmetic Mean (UPGMA)). It is defined as follows:
The idea is quite similar to complete linkage, but, in this case, the average of each cluster is taken into account and the goal is to minimize the average inter-cluster distance, considering all possible pairs (Ca, Cb). The following diagram shows an example of average linkage:
Example of average linkage. C1 and C2 are selected for merging. The highlighted points are the averages.
Average ...
