TheWard error sum of squares hierarchical clustering method has been very widely used since its rst description by Ward in a 1963 publication. It has also been generalized in various ways. However there are di erent interpretations in the literature and there are di erent implementations of the Ward agglomerative algorithm in commonly used software systems, including di ering 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. Ward’s Hierarchical Clustering Method: Clustering Criterion and Agglomerative Algorithm