Tim Finney has made available an article on-line "How To Discover Textual Groups"
This article describes how to discover groups among New Testament
witnesses using three different multivariate analysis (MVA) methods,
focussing on textual variation in the Gospel of Mark. It applies
statistical reasoning to the question of what level of agreement or
disagreement is required for one to be confident that two witnesses are
somehow related, whether in an adjacent or opposite sense. One MVA
method, PAM analysis, partitions witnesses into a specified number of
groups, identifies the most central member of each group, and identifies which numbers of groups are more natural for a data set. The MVA methods manage to identify many groups noticed by prior generations of scholars. One conclusion of the article is that Colwell and Tune's definition of a group (70% agreement plus 10% gap) should be abandoned.