With the single linkage method (also called nearest neighbor method), the distance between two clusters is the minimum distance between an observation in one cluster and an observation in the other cluster. When observations lie close together, single linkage tends to identify long chain-like clusters, with relatively large distances separating observations at either end of the chain.
The distance is calculated with the following distance matrix:
dmj = min (dkj, dlj)
|dmj||distance between clusters m and j|
|m||merged cluster that consists of clusters k and l, with m = (k,i)|
|dkj||distance between clusters k and j|
|dlj||distance between clusters l and j|