Ward's Hierarchical Clustering Method: Clustering Criterion and   Agglomerative Algorithm
  The Ward error sum of squares hierarchical clustering method has been very widely used since its first description by Ward in a 1963 publication. It has also been generalized in various ways. However there are different interpretations in the literature and there are different implementations of the Ward agglomerative algorithm in commonly used software systems, including differing expressions of the agglomerative criterion. Our survey work and case studies will be useful for all those involved in developing software for data analysis using Ward's hierarchical clustering method. 
Fionn Murtagh and Pierre Legendre
stat.ML cs.CV stat.AP