A Grounded Unsupervised Universal Part-of-Speech Tagger for Low-Resource Languages

Unsupervised part of speech (POS) tagging is often framed as a clustering problem, but practical taggers need to ground their clusters as well. Grounding generally requires reference labeled data, a luxury a low-resource language might not have. In this work, we describe an approach for low-resource unsupervised POS tagging that yields fully grounded output and requires no labeled training data. We find the classic method of Brown et al. (1992) clusters well in our use case and employ a decipherment-based approach to grounding. This approach presumes a sequence of cluster IDs is a'ciphertext' and seeks a POS tag-tocluster ID mapping that will reveal the POS sequence. We show intrinsically that, despite the difficulty of the task, we obtain reasonable performance across a variety of languages. We also show extrinsically that incorporating our POS tagger into a name tagger leads to stateof-the-art tagging performance in Sinhalese and Kinyarwanda, two languages with nearly no labeled POS data available. We further demonstrate our tagger's utility by incorporating Figure 1: Overview of our approach to grounded POS it into a true'zero-resource' variant of the tagging. We use an unsupervised clustering method MALOPA(Ammar et al., 2016) dependency (Section 3.2) then reduce and ground the clusters using parser model that removes the current reliance a decipherment approach informed by POS tag sequence on multilingual resources and gold POS tags data from many languages (Section 3.3).