  Traditional models for question answering optimize using cross entropy loss,
which encourages exact answers at the cost of penalizing nearby or overlapping
answers that are sometimes equally accurate. We propose a mixed objective that
combines cross entropy loss with self-critical policy learning. The objective
uses rewards derived from word overlap to solve the misalignment between
evaluation metric and optimization objective. In addition to the mixed
objective, we improve dynamic coattention networks (DCN) with a deep residual
coattention encoder that is inspired by recent work in deep self-attention and
residual networks. Our proposals improve model performance across question
types and input lengths, especially for long questions that requires the
ability to capture long-term dependencies. On the Stanford Question Answering
Dataset, our model achieves state-of-the-art results with 75.1% exact match
accuracy and 83.1% F1, while the ensemble obtains 78.9% exact match accuracy
and 86.0% F1.
