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Disclaimer: I'm no expert in the field, this question could be axiomatically flawed.

When programming a very basic regression based machine learning algorithm one uses a number of variables; it seems to me that when applying this to natural language processing the number of variables would be so large that the training set would be unreasonable. I've heard that a good rule of thumb for training set size is 5 - 15 times the number of variables - even with a relatively simple application that's many thousands of words - many millions of 2 word 'n-grams'.

Overall question: How would one apply machine learning when one of the inputs is a body of text?

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There are other techniques, but the simplest one is to start with dimensionality reduction - e.g. Principal Component Analysis. After reducing the dimensionality you can employ usual ML tools for doing classification, regression, etc.

Here are a few articles that may give you more information about the process

  • Martins, C. A., Monard, M. C., & Matsubara, E. T. (2003). Reducing the dimensionality of bag-of-words text representation used by learning algorithms. pdf
  • Shafiei, M. et al (2007). Document representation and dimension reduction for text clustering. pdf
  • Penagarikano, M., Varona, A., Rodríguez, L. J., & Bordel, G. (2011). Dimensionality Reduction for Using High-Order n-Grams in SVM-Based Phonotactic Language Recognition. pdf
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  • $\begingroup$ Thanks for the great answer. Any time I get an answer with references it's always more helpful! $\endgroup$ – FraserOfSmeg Jan 11 '15 at 14:35

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