Learning to Map into a Universal POS Tagset

We present an automatic method for mapping language-specific part-of-speech tags to a set of universal tags. This unified representation plays a crucial role in cross-lingual syntactic transfer of multilingual dependency parsers. Until now, however, such conversion schemes have been created manually. Our central hypothesis is that a valid mapping yields POS annotations with coherent linguistic properties which are consistent across source and target languages. We encode this intuition in an objective function that captures a range of distributional and typological characteristics of the derived mapping. Given the exponential size of the mapping space, we propose a novel method for optimizing over soft mappings, and use entropy regularization to drive those towards hard mappings. Our results demonstrate that automatically induced mappings rival the quality of their manually designed counterparts when evaluated in the context of multilingual parsing.1