by Peter Fenwick, Department of Computer Science, The University of Auckland.

In his work on the information content of English text in 1951, Shannon described a method of recoding the input text, a technique which has apparently lain dormant for the ensuing 45 years. Whereas traditional compressors exploit symbol frequencies and symbol contexts, Shannon’s method adds the concept of “symbol ranking”, as in `the next symbol is the one third most likely in the present context’.

http://www.jucs.org/jucs_3_2/symbol_ranking_text_compression