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Machine Learning, Part I: Supervised and Unsupervised Learning
(Up to General AI)

Machine Learning, Part II: Supervised and Unsupervised Learning

Last time, we discussed two types of learning that were based on the result of learning. This article will focus on another dimension to learning: whether it is supervised or unsupervised. Supervised learning is the type of learning that takes place when the training instances are labelled with the correct result, which gives feedback about how learning is progressing. This is akin to having a supervisor who can tell the agent whether or not it was correct. In unsupervised learning, the goal is harder because there are no pre-determined categorizations.

Supervised Learning

Supervised learning is fairly common in classification problems because the goal is often to get the computer to learn a classification system that we have created. Digit recognition, once again, is a common example of classification learning. More generally, classification learning is appropriate for any problem where deducing a classification is useful and the classification is easy to determine. In some cases, it might not even be necessary to give pre-determined classifications to every instance of a problem if the agent can work out the classifications for itself. This would be an example of unsupervised learning in a classification context.

Supervised learning is the most common technique for training neural networks and decision trees. Both of these techniques are highly dependent on the information given by the pre-determined classifications. In the case of neural networks, the classification is used to determine the error of the network and then adjust the network to minimize it, and in decision trees, the classifications are used to determine what attributes provide the most information that can be used to solve the classification puzzle. We'll look at both of these in more detail, but for now, it should be sufficient to know that both of these examples thrive on having some "supervision" in the form of pre-determined classifications.

Speech recognition using hidden Markov models and Bayesian networks relies on some elements of supervision as well in order to adjust parameters to, as usual, minimize the error on the given inputs.

Notice something important here: in the classification problem, the goal of the learning algorithm is to minimize the error with respect to the given inputs. These inputs, often called the "training set", are the examples from which the agent tries to learn. But learning the training set well is not necessarily the best thing to do. For instance, if I tried to teach you exclusive-or, but only showed you combinations consisting of one true and one false, but never both false or both true, you might learn the rule that the answer is always true. Similarly, with machine learning algorithms, a common problem is over-fitting the data and essentially memorizing the training set rather than learning a more general classification technique.

As you might imagine, not all training sets have the inputs classified correctly. This can lead to problems if the algorithm used is powerful enough to memorize even the apparently "special cases" that don't fit the more general principles. This, too, can lead to overfitting, and it is a challenge to find algorithms that are both powerful enough to learn complex functions and robust enough to produce generalizable results.

Unsupervised learning

Unsupervised learning seems much harder: the goal is to have the computer learn how to do something that we don't tell it how to do! There are actually two approaches to unsupervised learning. The first approach is to teach the agent not by giving explicit categorizations, but by using some sort of reward system to indicate success. Note that this type of training will generally fit into the decision problem framework because the goal is not to produce a classification but to make decisions that maximize rewards. This approach nicely generalizes to the real world, where agents might be rewarded for doing certain actions and punished for doing others.

Often, a form of reinforcement learning can be used for unsupervised learning, where the agent bases its actions on the previous rewards and punishments without necessarily even learning any information about the exact ways that its actions affect the world. In a way, all of this information is unnecessary because by learning a reward function, the agent simply knows what to do without any processing because it knows the exact reward it expects to achieve for each action it could take. This can be extremely beneficial in cases where calculating every possibility is very time consuming (even if all of the transition probabilities between world states were known). On the other hand, it can be very time consuming to learn by, essentially, trial and error.

But this kind of learning can be powerful because it assumes no pre-discovered classification of examples. In some cases, for example, our classifications may not be the best possible. One striking exmaple is that the conventional wisdom about the game of backgammon was turned on its head when a series of computer programs (neuro-gammon and TD-gammon) that learned through unsupervised learning became stronger than the best human chess players merely by playing themselves over and over. These programs discovered some principles that surprised the backgammon experts and performed better than backgammon programs trained on pre-classified examples.

A second type of unsupervised learning is called clustering. In this type of learning, the goal is not to maximize a utility function, but simply to find similarities in the training data. The assumption is often that the clusters discovered will match reasonably well with an intuitive classification. For instance, clustering individuals based on demographics might result in a clustering of the wealthy in one group and the poor in another.

Although the algorithm won't have names to assign to these clusters, it can produce them and then use those clusters to assign new examples into one or the other of the clusters. This is a data-driven approach that can work well when there is sufficient data; for instance, social information filtering algorithms, such as those that Amazon.com use to recommend books, are based on the principle of finding similar groups of people and then assigning new users to groups. In some cases, such as with social information filtering, the information about other members of a cluster (such as what books they read) can be sufficient for the algorithm to produce meaningful results. In other cases, it may be the case that the clusters are merely a useful tool for a human analyst. Unfortunately, even unsupervised learning suffers from the problem of overfitting the training data. There's no silver bullet to avoiding the problem because any algorithm that can learn from its inputs needs to be quite powerful.

Summary

Unsupervised learning has produced many successes, such as world-champion calibre backgammon programs and even machines capable of driving cars! It can be a powerful technique when there is an easy way to assign values to actions. Clustering can be useful when there is enough data to form clusters (though this turns out to be difficult at times) and especially when additional data about members of a cluster can be used to produce further results due to dependencies in the data.

Classification learning is powerful when the classifications are known to be correct (for instance, when dealing with diseases, it's generally straight-forward to determine the design after the fact by an autopsy), or when the classifications are simply arbitrary things that we would like the computer to be able to recognize for us. Classification learning is often necessary when the decisions made by the algorithm will be required as input somewhere else. Otherwise, it wouldn't be easy for whoever requires that input to figure out what it means.

Both techniques can be valuable and which one you choose should depend on the circumstances--what kind of problem is being solved, how much time is allotted to solving it (supervised learning or clustering is often faster than reinforcement learning techniques), and whether supervised learning is even possible.

Continue to Machine Learning, Part III: Testing algorithms and the "No Free Lunch" theorem

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