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I have datasets that range from ~2000-9000 columns of predictor variables. I'm often charged with primarily classification - but sometimes regression tasks. I know that I don't need this many variables for my models to be effective but I can't anticipate which ones in a reliable way.

I'm looking for ideas on the general best practices that would cut this down to around 50-150 variables which from my experience, seems fairly effective in determining the outcome.

Currently I'm using lasso or random forest to whittle down the number of variables before running a final model. I want less variables so theres less noise, simply don't need that many, and to make it easier to deploy to production.

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    $\begingroup$ Fewer can be effective if you know without looking at your data which few that represents. Features selection based on relationships with $Y$ can be double dipping and will result in instability and arbitrary selection of variables in many cases. Global penalized regression is often preferred. $\endgroup$ – Frank Harrell Jul 20 '17 at 22:37
  • $\begingroup$ @FrankHarrell +1 for your second and third sentence but what do you mean with your first sentence? It is slightly more cryptic than thought-provoking... $\endgroup$ – usεr11852 Aug 5 '17 at 11:21
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    $\begingroup$ I was referring to the illusion of parsimony and misuse of Occam's razor in effect. Fewer is better if the features are pre-specified. If the feature subset is obtained by data torture, all bets are off. As I explain in my course notes from biostat.mc.vanderbilt.edu/rms there is an analogy to Maxwell's Demon where predictive information is "stolen" from the system by attempting to find which predictors to use to predict. $\endgroup$ – Frank Harrell Aug 5 '17 at 12:20
  • $\begingroup$ Makes sense now, thank you for your clarifications! $\endgroup$ – usεr11852 Aug 5 '17 at 12:30
  • $\begingroup$ Regarding the idea of using a random forest model first, you might be interested in reading this: Can a random forest be used for feature selection in multiple linear regression? (You would also need to use a randomly selected subset as the training set, & then fit your 'real' model w/ the selected variables on the hold-out set.) $\endgroup$ – gung - Reinstate Monica Jan 22 '18 at 21:17
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I think you are doing a "best practice" approach already for feature selection. Using a regularised regression approach like LASSO and complementing those insights with a distribution-free model like Random Forest to decide the most important features is probably the best way to go.

Some minor suggestions: I would propose using Elastic Net to potentially have a small amount of $L_2$ regularisation. This should make our feature selection a bit more stable in case of correlated features. Similarly, taking a slightly more sophisticated approach of using Random Forests within a full Recursive Feature Elimination framework like Baruta (See Nilsson et al. for background, CRAN link) instead of simply relying on simple Random Forest variable importance will probably be beneficial.

Having said the above, we should use such feature selection approaches only if we cannot work with our original full dataset and/or we have problem collecting the features in question in the future (eg. too costly). Using a modelling approach that can actively regularise the resulting model (eg. gradient boosting machines where we can regularise the fit by properly picking the learning rate, tree-depth, minimum number of children per leaf node, etc.) is the best way to go. In that way we know we are not reusing our data, as well as that we are not losing valuable information that was potentially missed out during our feature selection step.

An issue, not touched upon is performing data reduction using some dimensionality reduction technique like PCA, ICA, NNMF, etc. These techniques do not "select features" per se but rather "combine features" to create meta-features of variable informational value. They can be very useful if we need a small subset of "information-rich" features. Nevertheless, these "information-rich" features are not guarantee to include more, less or any information relevant to our modelling task so they are not a silver bullet for feature selection. They usually present a convenient and condensed representation of our original data when we cannot work with their raw form.

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