Text Classification using String Kernels

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Text Classification using String Kernels

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by Huma Lodhi , Craig Saunders , John Shawe-Taylor , Nello Cristianini , Chris Watkins

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495 - 7 self

BibTeX

@MISC{Lodhi_textclassification,
    author = {Huma Lodhi and Craig Saunders and John Shawe-Taylor and Nello Cristianini and Chris Watkins},
    title = {Text Classification using String Kernels},
    year = {}
}

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Abstract

We propose a novel approach for categorizing text documents based on the use of a special kernel. The kernel is an inner product in the feature space generated by all subsequences of length k. A subsequence is any ordered sequence of k characters occurring in the text though not necessarily contiguously. The subsequences are weighted by anexponentially decaying factor of their full length in the text, hence emphasising those occurrences that are close to contiguous. A direct computation of this feature vector would involve a prohibitive amount of computation even for modest values of k, since the dimension of the feature space grows exponentially with k. The paper describes how despite this fact the inner product can be e ciently evaluated by a dynamic programming technique. Experimental comparisons of the performance of the kernel compared with a standard word feature space kernel Joachims (1998) show positive results on modestly sized datasets. The case of contiguous subsequences is also considered for comparison with the subsequences kernel with di erent decay factors. For larger documents and datasets the paper introduces an approximation technique that is shown to deliver good approximations e ciently for large datasets.

Keyphrases

text classification string kernel feature space inner product large datasets feature vector prohibitive amount di erent decay factor good approximation full length contiguous subsequence dynamic programming technique text document special kernel modest value novel approach experimental comparison positive result approximation technique direct computation ordered sequence

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