Open-domain question answering can be reformulated as a phrase retrieval problem, without the need for processing documents on-demand during inference (Seo et al., 2019).
However, current phrase retrieval models heavily depend on sparse representations and still underperform retriever-reader approaches.
In this work, we show for the first time that we can learn dense representations of phrases alone that achieve much stronger performance in open-domain QA.
We present an effective method to learn phrase representations from the supervision of reading comprehension tasks, coupled with novel negative sampling methods.
We also propose a query-side fine-tuning strategy, which can support transfer learning and reduce the discrepancy between training and inference.
On five popular open-domain QA datasets, our model DensePhrases improves over previous phrase retrieval models by 15%-25% absolute accuracy and matches the performance of state-of-the-art retriever-reader models.
Our model is easy to parallelize due to pure dense representations and processes more than 10 questions per second on CPUs.
Finally, we directly use our pre-indexed dense phrase representations for two slot filling tasks, showing the promise of utilizing DensePhrases as a dense knowledge base for downstream tasks.

Learning Dense Representations of Phrases at Scale

**Session Chair: ** Sara Rosenthal 
**Session Volunteer: ** Oyeniran Oluwashina Akinloye

14E: Question Answering #4

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Learning Dense Representations of Phrases at Scale

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