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documents can be stored as closely together as possible. The early work of Ivie[76] was similarly motivated in that he proposed to collect feedback information from users showing which pairs of documents were frequently was then taken as proportional to the strength of association, and documents more closely associated were made more readily accessible than those less closely associated.

Finally, the reader may be interested in pursuing the use of cluster methods in pattern recognition since some of the ideas developed there are applicable to IR. Both Duda and Hart[77]and Watanabe[78] devote a chapter to clustering in the context of pattern recognition.

References

1. KENDALL, M.G., In Multivariate Analysis (Edited by P.R. Krishnaiah), Academic Press, London and New York, 165-184 (1966).

2. JARDINE, N. and SIBSON, R., Mathematical Taxonomy, Wiley, London and New York (1971).

3. MACNAUGHTON-SMITH, P., Some Statistical and Other Numerical Techniques for Classifying Individuals, Studies in the causes of delinquency and the treatment of offenders. Report No. 6, HMSO, London (1965).

4. HEMPEL, C.G., Aspects of Scientific Explanation and Other Essays in the Philosophy of Science, The Free Press, New York, 137-154 (1965).

5. SPARCK JONES, K., Automatic Keyword Classification for Information Retrieval, Butterworths, London (1971).

6. GOOD, I.J., Speculations Concerning Information Retrieval, Research Report PC-78, IBM Research Centre, Yorktown Heights, New York (1958).

7. FAIRTHORNE, R.A., 'The mathematics of classification'. Towards Information Retrieval, Butterworths, London, 1-10 (1961).

8. HAYES, R.M., 'Mathematical models in information retrieval'. In Natural Language and the Computer (Edited by P.L. Garvin), McGraw-Hill, New York, 287 (1963).

9. SALTON, G., Automatic Information Organization and Retrieval, McGraw-Hill, New York, 18 (1968).

10. JARDINE, N. and van RIJSBERGEN, C.J., 'The use of hierarchic clustering in information retrieval', Information Storage and Retrieval, 7, 217-240 (1971).

11. GOODMAN, L. and KRUSKAL, W., 'Measures of association for cross-classifications', Journal of the American Statistical Association, 49, 732-764 (1954).

12. GOODMAN, L. and KRUSKAL, W., 'Measures of association for cross-classifications II: Further discussions and references', Journal of the American Statistical Association, 54, 123-163 (1959).

13. KUHNS, J.L., 'The continuum of coefficients of association'. In Statistical Association Methods for Mechanised Documentation, (Edited by Stevens et al.) National Bureau of Standards, Washington, 33-39 (1965).

14. CORMACK, R.M., 'A review ofclassification', Journal of the Royal Statistical Society, Series A,134, 321-353 (1971).