user: text: Write a concise one-sentence TLDR summary of the scientific paper content below.

Title: Evidence Aggregation for Answer Re-Ranking in Open-Domain Question Answering

Paper content:
Very recently, it comes to be a popular approach for answering open-domain questions by first searching question-related passages, then applying reading comprehension models to extract answers.
Existing works usually extract answers from single passages independently, thus not fully make use of the multiple searched passages, especially for the some questions requiring several evidences, which can appear in different passages, to be answered.
The above observations raise the problem of evidence aggregation from multiple passages.
In this paper, we deal with this problem as answer re-ranking.
Specifically, based on the answer candidates generated from the existing state-of-the-art QA model, we propose two different re-ranking methods, strength-based and coverage-based re-rankers, which make use of the aggregated evidences from different passages to help entail the ground-truth answer for the question.
Our model achieved state-of-the-arts on three public open-domain QA datasets, Quasar-T, SearchQA and the open-domain version of TriviaQA, with about 8\% improvement on the former two datasets.
Open-domain question answering (QA) aims to answer questions from a broad range of domains by effectively marshalling evidence from large open-domain knowledge sources.
Such resources can be Wikipedia , the whole web BID12 , structured knowledge bases BID2 or combinations of the above (Baudiš &Šedivỳ, 2015) .Recent
work on open-domain QA has focused on using unstructured text retrieved from the web to build machine comprehension models BID9 . These
studies adopt a two-step process: an information retrieval (IR) model to coarsely select passages relevant to a question, followed by a reading comprehension (RC) model BID26 to infer an answer from the passages. These
studies have made progress in bringing together evidence from large data sources, but they predict an answer to the question with only a single retrieved passage at a time. However
, answer accuracy can often be improved by using multiple passages. In some
cases, the answer can only be determined by combining multiple passages.In this paper, we propose a method to improve open-domain QA by explicitly aggregating evidence from across multiple passages. Our method
is inspired by two notable observations from previous open-domain QA results analysis:• First, compared with incorrect answers, the correct answer is often suggested by more passages repeatedly. For example
, in FIG0 (a), the correct answer "danny boy" has more passages providing evidence relevant to the question compared to the incorrect one. This observation
can be seen as multiple passages collaboratively enhancing the evidence for the correct answer.• Second, sometimes
the question covers multiple answer aspects, which spreads over multiple passages. In order to infer the
correct answer, one has to find ways to aggregate those multiple passages in an effective yet sensible way to try to cover all aspects. In FIG0 the correct answer
"Galileo Galilei" at the bottom has passages P1, "Galileo was a physicist ..." and P2, "Galileo discovered the first 4 moons of Jupiter", mentioning two pieces of evidence to match the question. In this case, the aggregation
of these two pieces of evidence can help entail the ground-truth answer "Galileo Galilei". In comparison, the incorrect
answer "Isaac Newton" has passages providing partial evidence on only "physicist, mathematician and astronomer". This observation illustrates
the way in which multiple passages may provide complementary evidence to better infer the correct answer to a question.To provide more accurate answers for open-domain QA, we hope to make better use of multiple passages for the same question by aggregating both the strengthened and the complementary evidence from all the passages. We formulate the above evidence
aggregation as an answer re-ranking problem. Re-ranking has been commonly used
in NLP problems, such as in parsing and translation, in order to make use of high-order or global features that are too expensive for decoding algorithms BID6 BID27 BID16 BID11 . Here we apply the idea of re-ranking
; for each answer candidate, we efficiently incorporate global information from multiple pieces of textual evidence without significantly increasing the complexity of the prediction of the RC model. Specifically, we first collect the
top-K candidate answers based on their probabilities computed by a standard RC/QA system, and then we use two proposed re-rankers to re-score the answer candidates by aggregating each candidate's evidence in different ways. The re-rankers are:• A strength-based
re-ranker, which ranks the answer candidates according to how often their evidence occurs in different passages. The re-ranker is based on the first observation
if an answer candidate has multiple pieces of evidence, and each passage containing some evidence tends to predict the answer with a relatively high score (although it may not be the top score), then the candidate is more likely to be correct. The passage count of each candidate, and the aggregated
probabilities for the candidate, reflect how strong its evidence is, and thus in turn suggest how likely the candidate is the corrected answer.• A coverage-based re-ranker, which aims to rank an answer
candidate higher if the union of all its contexts in different passages could cover more aspects included in the question. To achieve this, for each answer we concatenate all the passages
that contain the answer together. The result is a new context that aggregates all the evidence necessary
to entail the answer for the question. We then treat the new context as one sequence to represent the answer,
and build an attention-based match-LSTM model between the sequence and the question to measure how well the new aggregated context could entail the question. Overall, our contributions are as follows: 1) We propose a re-ranking-based
framework to make use of the evidence from
multiple passages in open-domain QA, and two re-rankers, namely, a strengthbased re-ranker and a coverage-based re-ranker, to perform evidence aggregation in existing opendomain QA datasets. We find the second re-ranker performs better than the first one on two of the
three public datasets. 2) Our proposed approach leads to the state-of-the-art results on three different
datasets (Quasar-T BID9 , SearchQA BID10 and TriviaQA BID17 ) and outperforms previous state of the art by large margins. In particular, we achieved up to 8% improvement on F1 on both Quasar-T and SearchQA
compared to the previous best results.
We have observed that open-domain QA can be improved by explicitly combining evidence from multiple retrieved passages.
We experimented with two types of re-rankers, one for the case where evidence is consistent and another when evidence is complementary.
Both re-rankers helped to significantly improve our results individually, and even more together.
Our results considerably advance the state-of-the-art on three open-domain QA datasets.Although our proposed methods achieved some successes in modeling the union or co-occurrence of multiple passages, there are still much harder problems in open-domain QA that require reasoning and commonsense inference abilities.
In future work, we will explore the above directions, and we believe that our proposed approach could be potentially generalized to these more difficult multipassage reasoning scenarios.

assistant: text: We propose a method that can make use of the multiple passages information for open-domain QA.