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Do We Need Hundreds of Classifiers to Solve Real World Classification Problems? [pdf] (mit.edu)
108 points by msherry on Dec 8, 2014 | hide | past | web | favorite | 7 comments

This is consistent with our experience running hundreds of Kaggle competitions: for most classification problems, some variation on ensembled decision trees (random forests, gradient boosted machines, etc.) performs the best. This is typically in conjunction with clever data processing, feature selection, and internal validation.

One key exception is where the data is richly and hierarchically structured. Text, speech, and visual data falls under this category. In many cases here, variations of neural networks (deep neural nets/CNN's/RNN's/etc.) provide very dramatic improvements.

This study does have a couple limitations. The datasets used are very small & form a very biased selection of real-world applications of machine learning. It doesn't consider ensembles of different model types (which I'd expect to provide a consistent but marginal improvement over the results here).

Great commentary. Encoding high-level categorical variables as unordered integers, or even frequency count maps, is pretty common practice, but really only valid (as the author says) in tree based methods. Given deep enough trees (or enough iterations in the boosting scenario), it is my anecdotal experience that trees are able to parse out relevant values. The problem with indicator variables in general is the enormous potential loss in computational time and increase in complexity. My models have hundreds of vehicle/car types, U.S. state, etc., and so this type of coding helps quite a bit in performance without sacrificing (and sometimes improving due to curse-of-dimensionality) model performance (IMHO YMMV)

From the commentary: "we are always more accepting of an expected outcome", which I think says a lot. Random Forests are great, and work well, but sometimes it feels like there's a world of difference between Kaggle-contest-style short burst projects and long-term "we need maximum accuracy" projects.

Other approaches always have a place, and the skill is in knowing when they should be deployed.

I think the lesson here has been evident for some time: There is no one best classifier, only classifiers that perform better on particular problems. Random Forests work well because as ensemble methods they can be optimized to explore many different aspects of a data set, but they don't perform significantly better than SVMs or most of the other methods.

In practice the solution is often to use a combination of multiple methods. Trees, support vector machines, multilayer perceptrons, gaussian kernels, bagging and boosting. In most applications you don't have choose. Combining the results of all of them together using a weighted average will out perform any of them individually. And in most cases, the whole is greater than the sum of its parts. Each classifier fits a given data set differently and provides its own perspective on the prediction problem. The goal isn't to choose the best one, but to find an ensemble of methods that best explain the patterns and relationships in the data.

There are many cases where resource and speed limitations dictate that only one classifier can be tuned and implemented, and in those situations it's good to know which one is 'best'. But when it's possible to build an ensemble out of many different methods it's almost always the best way to go.

Couldn't agree with this comment more. This hit home for me when I read Grimmer and King's "General Purpose Computer-Assisted Clustering and Conceptualization" (http://gking.harvard.edu/files/gking/files/201018067_online....), which looked at every text clustering/classification alogithm they could find at the time (as summarized http://gking.harvard.edu/files/gking/files/discov-supp.pdf). They basically end up clustering in the space of clustering algorithms...

>I think the lesson here has been evident for some time: There is no one best classifier, only classifiers that perform better on particular problems.

Isn't that called the No Free Lunch Theorem?

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