Cluster is sometimes called unsupervised learning. Single link - A
near node was allowed, should I. VS Complete link - I can't let
another in with much better nodes. Purity if cluster problem - Because
we could specify number of clusters equal to the number of elements,
and this would give us perfect purity! This can only work if we
enforce a number of clusters. Creating an optimal clustering is not
always simple due to the large number of permutations, instead we
create an intuitive cluster and then iteratively improve it. - Thomas
Hayden
