Clustering variables is essentially the same thing as proximity variable processing except that proximity variable processing operates on two words and clustering variables operates on three or more words. On occasion it is useful to find where clusters of words are, rather than just to find a word or two. Finding clusters of words in close proximity is called cluster analysis.
In cluster analysis the contents of one or more documents are gathered, and an analysis of the frequency of the words in the document and the proximity of those words to each other is performed. A cluster analysis is done strictly on the basis of the contents of the specific document or documents being analyzed.