Orthogonal sparse bigrams are another interesting tokenization method, the authors of this paper tested it out against CRM114 with good results:<p><a href="http:&#x2F;&#x2F;www.siefkes.net&#x2F;ie&#x2F;winnow-spam.pdf" rel="nofollow">http:&#x2F;&#x2F;www.siefkes.net&#x2F;ie&#x2F;winnow-spam.pdf</a>