On Kernel Methods for Relational Learning

Kernel methods have gained a great deal of popularity in the machine learning community as a method to learn indirectly in high-dimensional feature spaces. Those interested in relational learning have recently begun to cast learning from structured and relational data in terms of kernel operations. We describe a general family of kernel functions built up from a description language of limited expressivity and use it to study the benefits and drawbacks of kernel learning in relational domains. Learning with kernels in this family directly models learning over an expanded feature space constructed using the same description language. This allows us to examine issues of time complexity in terms of learning with these and other relational kernels, and how these relate to generalization ability. The tradeoffs between using kernels in a very high dimensional implicit space versus a restricted feature space, is highlighted through two experiments, in bioinformatics and in natural language processing.