differences in the scale and in the use to which a classification structure is to be put.
In the case of scale, the size of the problem in IR is invariably such that for cluster methods based on similarity matrices it becomes impossible to store the entire similarity matrix, let alone allow random access to its elements.
If we are to have a reasonably useful cluster method based on similarity matrices we must be able to generate the similarity matrix in small sections, use each section to update the classification structure immediately after it has been generated and then throw it away.
The importance of this fact was recognised by Needham.
van Rijsbergen has described an implementation of single-link which satisfies this requirement.
When a classification is to be used in IR, it affects the design of the algorithm to the extent that a classification will be represented by a file structure which is
(1) easily updated;
(2) easily searched; and
(3) reasonably compact.
Only (3) needs some further comment.
It is inevitable that parts of the storage used to contain a classification will become redundant during an updating phase.
This being so it is of some importance to be able to reuse this storage, and if the redundant storage becomes excessive to be able to process the file structure in such a way that it will subsequently reside in one contiguous part of core.
This 'compactness' is particularly important during experiments in which the file structure is read into core before being accessed.
Let me briefly summarise the logical structure of this chapter.
It started very generally with a descriptive look at automatic classification and its uses.
It then discussed association measures which form the basis of an important class of classification methods.
Next came a breakdown of classification methods.
This was followed by a statement of the hypothesis underlying the use of automatic classification in document clustering.
It went on to examine in some detail the use of classification methods in IR leading up to recommendation of single-link for document clustering.
Finally we made some practical points about implementation.
This chapter ended on a rather practical note.
We continue in this vein in the next chapter where we discuss file structures.
These are important if we are to appreciate how it is that we can get dictionaries,