PCA before random forest classification

Does it make sense to do PCA before carrying out a Random Forest Classification?

I'm dealing with high dimensional text data, and I want to do feature reduction to help avoid the curse of dimensionality, but don't Random Forests already to some sort of dimension reduction?

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The RF algorithm does not really suffer from high number of predictors since it only take a random subset of them (so called mtry parameter) to build each tree. There is also a recursive feature elimination technique built on top of the RF algorithm (see the varSelRF R package and references therein). It is, however, certainly possible to add an initial data reduction scheme, although it should be part of the cross-validation process. So the question is: do you want to input a linear combination of your features to RF? –  chl Jan 10 '13 at 21:03

Leo Brieman wrote that "dimensionality can be a blessing". In general, random forests can run on large data sets without problems. How large is your data? Different fields handle things in different ways depending on subject-matter knowledge. For example, in gene expression studies genes are often discarded based on low variance (no peeking at the outcome) in a process sometimes called non-specific filtering. This can help with the running time on random forests. But it is not required.

Sticking with the gene expression example, sometimes analysts use PCA scores to represent gene expression measurements. The idea is to replace similar profiles with one score that is potentially less messy. Random forests can be run both on the original variables or the PCA scores (a surrogate for the variables). Some have reported better results with this approach, but there are no good comparisons to my knowledge.

In sum, there is no need to do PCA before running RF. But you can. The interpretation could change depending on your goals. If all you want to do is predict, the interpretation may be less important.

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Thank you for the response. Speed is an issue, more because I have several thousand possible labels in a multi-label problem. The application is classifying a corpus of text data drawn from both twitter and analysts' description of certain events. I'm using tf-idf weighting and the bag of words model. –  Maus Jan 10 '13 at 22:04

PCA before random forest can be useful not for dimensionality reduction but to give you data a shape where random forest can perform better.

I am quiet sure that in general if you transform your data with PCA keeping the same dimensionality of the original data you will have a better classification with random forest

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