Momentum Posterior Regularization for Multi-hop Dense Retrieval
This work addresses a specific bottleneck in multi-hop retrieval for question answering, offering an incremental improvement over prior methods.
The paper tackled the problem of knowledge distillation for multi-hop dense retrieval in question answering, where existing methods are ineffective due to unclear posterior information and large knowledge gaps, and proposed MoPo with query-focused summaries and momentum-based training, resulting in outperformance over baselines on HotpotQA and StrategyQA datasets.
Multi-hop question answering (QA) often requires sequential retrieval (multi-hop retrieval), where each hop retrieves missing knowledge based on information from previous hops. To facilitate more effective retrieval, we aim to distill knowledge from a posterior retrieval, which has access to posterior information like an answer, into a prior retrieval used during inference when such information is unavailable. Unfortunately, current methods for knowledge distillation in one-time retrieval are ineffective for multi-hop QA due to two issues: 1) Posterior information is often defined as the response (i.e. the answer), which may not clearly connect to the query without intermediate retrieval; and 2) The large knowledge gap between prior and posterior retrievals makes existing distillation methods unstable, even resulting in performance loss. As such, we propose MoPo (Momentum Posterior Regularization) with two key innovations: 1) Posterior information of one hop is defined as a query-focus summary from the golden knowledge of the previous and current hops; 2) We develop an effective training strategy where the posterior retrieval is updated along with the prior retrieval via momentum moving average method, allowing smoother and effective distillation. Experiments on HotpotQA and StrategyQA demonstrate that MoPo outperforms existing baselines in both retrieval and downstream QA tasks.