which is the only one to have extensively used in document retrieval. It satisfies all the criteria of adequacy mentioned above. In fact, Jardine and Sibson[2] have shown that under a certain number of reasonable conditions single-link is the only hierarchic method satisfying these important criteria. It will be discussed in some detail in the next section.

A further class of cluster methods based on measurement of similarity is the class of so-called 'clump' methods. They proceed by seeking sets which satisfy certain cohesion and isolation conditions defined in terms of the similarity measure. The computational difficulties of this approach have largely caused it to be abandoned. An attempt to generate a hierarchy of clumps was made by van