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The Ratkowsky and Lance criterion for determining the "optimum" number of groups in a dendrogram is applicable to both nominal and numeric data, is easy to calculate, and requires no prior knowledge of the identity of the operational taxonomic units. A modification of that criterion, described in this paper, maintains the advantages of the original criterion and overcomes the problems of the invalid use of phenon lines and lack of objectivity. Preliminary work also suggests that the modification gives improved results for the number of groups.
Robert S. Hill (Mon,) studied this question.