Model Invertibility Regularization: Sequence Alignment With or Without Parallel Data

We present Model Invertibility Regularization (MIR), a method that jointly trains two directional sequence alignment models, one in each direction, and takes into account the invertibility of the alignment task. By coupling the two models through their parameters (as opposed to through their inferences, as in Liang et al.'s Alignment by Agreement (ABA), and Ganchev et al.'s Posterior Regularization (PostCAT)), our method seamlessly extends to all IBMstyle word alignment models as well as to alignment without parallel data. Our proposed algorithm is mathematically sound and inherits convergence guarantees from EM. We evaluate MIR on two tasks: (1) On word alignment, applying MIR on fertility based models we attain higher F-scores than ABA and PostCAT.