Title: Supervised Metric Learning with Generalization Guarantees,
Abstract: The crucial importance of metrics in machine learning algorithms has led to
an increasing interest in optimizing distance and similarity functions, an area
of research known as metric learning. When data consist of feature vectors, a
large body of work has focused on learning a Mahalanobis distance. Less work
has been devoted to metric learning from structured objects (such as strings or
trees), most of it focusing on optimizing a notion of edit distance. We
identify two important limitations of current metric learning approaches.
First, they allow to improve the performance of local algorithms such as
k-nearest neighbors, but metric learning for global algorithms (such as linear
classifiers) has not been studied so far. Second, the question of the
generalization ability of metric learning methods has been largely ignored. In
this thesis, we propose theoretical and algorithmic contributions that address
these limitations. Our first contribution is the derivation of a new kernel
function built from learned edit probabilities. Our second contribution is a
novel framework for learning string and tree edit similarities inspired by the
recent theory of (e,g,t)-good similarity functions. Using uniform stability
arguments, we establish theoretical guarantees for the learned similarity that
give a bound on the generalization error of a linear classifier built from that
similarity. In our third contribution, we extend these ideas to metric learning
from feature vectors by proposing a bilinear similarity learning method that
efficiently optimizes the (e,g,t)-goodness. Generalization guarantees are
derived for our approach, highlighting that our method minimizes a tighter
bound on the generalization error of the classifier. Our last contribution is a
framework for establishing generalization bounds for a large class of existing
metric learning algorithms based on a notion of algorithmic robustness.