On structured learning

This post is devoted to structured learning, a novel machine learning paradigm which may be used for solving different computer vision problems. For example, finding optimal parameters of graphical models is essentially a structured learning problem. I am going to give an introductory talk on structured learning for Dmitry Vetrov's graphical models class this Friday at 4:20 pm, so please feel free to drop by if you are in Moscow (the talk is to be in Russian).

Structured learning is basically a very general supervised learning setting. Consider the classification setting as a basic problem. One needs to find parameters of a function that maps a feature vector to one of the pre-defined class labels $\mathbb{R}^m \to \{c_1, c_2, \dots, c_K\}$. The fundamental property is the classes are unordered and orthogonal. The latest means the probabilities of objects classified as different labels are uncorrelated (some negative correlation is normal since strong classifying of an object as $c_i$ decreases the probability for rest of the labels).

Now consider regression, which is another supervised learning problem where feature vectors map to real values: $\mathbb{R}^m \to \mathbb{R}$. It might be considered a classification problem with infinite number of classes. There are two obvious flaws in this reduction. First, a training set is unlikely to have at least one example for each class. To overcome this one can quantize the codomain and train a classifier over a finite set of class labels. However, it leaves us with the second flaw: the method does not take into account correlation between the possible outcomes. The bins are ordered: the training features that correspond to neighbouring bins should be handled differently from those that correspond to distant bins. That's why some global methods (like linear regression) are used.

The similar situation is in the case of a structured outcome. In this case, one usually has a plenty of possible outcomes, but they are not independent. The techniques of structured learning are applicable when the outcomes have some underlying structure, and the elements of the structure have similar sets of features. Also, it should be possible to estimate how bad the prediction is (often in terms of incorrectly predicted elements), which is called structured loss. The methods allow small deviations from the ideal training outcome, which are possible e.g. because of noise, but penalize the substantial ones (just like regression!). The prediction function (parameters of which are tuned) can thus be formalized as a mapping $\mathbb{R}^{m \times l} \to \{c_1, c_2, \dots, c_K\}^l$.

The possible example is hidden Markov model learning, where the elements are emission potentials and transition potentials, and the loss can be defined as the number of wrong HMM outputs. According to the upper formalization, there are $l$ elements, each represented by $m$ features (in practice, $m$ can vary for different types of elements). Since the labels of transition elements are strictly defined by emission outputs, not every outcome is possible.

Another example of a structured prediction problem is natural language parsing. Given a sentence of a natural language, the corresponding parsing tree is to be constructed. Surprisingly, parsing trees could also be represented as high-dimensional vectors, with constraints applied. To summarize, structure prediction has outcome that is multivariate, correlated and constrained.

Ideally, the parameters of structured prediction should be tuned via likelihood maximization, but this turns out to be intractable due to the need of computing the partition function on each gradient optimization step. That's why the L2-regularized hinge loss is usually minimized. The prediction algorithm is represented as a scoring function over possible outcomes. The task of the learning algorithm is to find the parameters of the scoring function to make it return maximum values for the true outcomes on the training set, and low ones for the instances that are far from optimal. Given that, the margin between good and bad possible outcomes should be maximized (in terms of scoring function value).