Extractive document summarization
Extractive document summarization, as a challenging instance of natural language generation (NLG), is a popular summarisation paradigm, which builds summaries by selecting an appropriate sequence of important phrases or sentences from the input document(s). Extractive summarisation can be formulated as a sequential decision-making problem, and hence can be tackled by the Reinforcement Learning (RL) algorithms. RL searches for the (near-)optimal trajectories (i.e. sequences of decisions) by directly optimising the objective functions, e.g. the ROUGE metrics [Lin2004]
. Such objectives are non-differentiable and therefore difficult to be directly optimised by deep neural networks. In addition, RL alleviates theexposure bias
problem faced by sequential supervised learning paradigms in NLG. Combining RL algorithms such as REINFORCE[Williams1992] with neural techniques (e.g. sequence-to-sequence) yields state-of-the-art performance in summarisation [Narayan et al.2018, Yao et al.2018].
Existing RL-based summarisation systems fall into two categories: cross-input RL and input-specific RL. For cross-input RL (upper part in Fig. 1), at training time, RL agents interact with a ground-truth reward oracle for multiple episodes so as to learn a policy that maximises the accumulated rewards in the episode; at test time, the learnt policy is applied to unseen data to generate the summary. However, learning such cross-input policies is very expensive because of the huge search space. Another issue is the delayed rewards: the ROUGE scores can be calculated only when the complete summary is generated. Such delayed rewards cause RL-based summarisers to take even longer time to converge. Although multiple techniques have been proposed to speed up the training of RL, e.g. memory replay [Mnih et al.2015], MIXER [Ranzato et al.2016]Cheng et al.2018], training cross-input RL yet requires considerable time, data and parameter tuning.
On the other hand, input-specific RL (middle part in Fig. 1) neither requires parallel data (i.e. input documents and reference summaries for them) nor a reward oracle. Instead, it employs a handcrafted reward function with which the RL agent interacts in order to learn a policy specifically for the given input. By doing so, the size of the search space significantly decreases, which consequently diminishes the training time and computational resources. However, designing such a reward function is highly challenging as it should fit all inputs [Rioux et al.2014]. This explains the poor performance of input-specific RL summarisers [Ryang and Abekawa2012].
To tackle the problems faced by the above two RL-based summarisation paradigms, we propose a novel paradigm called REward Learning for Input-Specific reinforcement learning (RELIS). Instead of learning a cross-input policy, RELIS learns a cross-input reward oracle at training time, and then uses the learnt reward to train an input-specific policy for each input at test time (bottom part in Fig. 1). RELIS is inspired by inverse RL [Abbeel and Ng2004], which requires a demonstrator to present optimal trajectories. Because such a demonstrator is hardly available in summarisation, RELIS leverages Learning-to-Rank (L2R) algorithms [Li2011] to approximate the ground-truth reward oracle from “weak supervisions”, e.g., numeric scores that indicate the quality of the summary or preferences over summary pairs.
Our contributions are threefold: (i) We propose RELIS (§2), a new RL-based summarisation framework that enjoys the strong performance of cross-input RL and the low computational cost of input-specific RL. (ii) Theoretically, we prove that by employing appropriate L2R and RL algorithms, RELIS guarantees to generate near-optimal outputs (§3). (iii) Empirically, we evaluate RELIS in multi-document extractive summarisation (§4). Compared to the state-of-the-art methods, RELIS provides comparable performance but requires much less the training time or data. Because the proof of RELIS is generic, we believe RELIS has the potential to be applied to other NLG tasks, e.g. translation and sentence simplification. Source code and supplementary material are available at https://github.com/UKPLab/ijcai2019-relis.
We first formally define the summarisaiton task, and then detail the L2R and RL module of RELIS.
2.1 Extractive Summarisation
In line with DBLP:conf/acl/PeyrardE17a [DBLP:conf/acl/PeyrardE17a], we formulate summarisation as a discrete optimisation problem. Let be the set of all possible input documents and be the set of inputs available at training time. An input can be either a single document or a cluster of documents on the same topic. For input , let indicate the set of all extractive summaries for that comply with the length requirement. Then, the task of summarisation is to map each input to its best summary in with respect to a ranking function . For a candidate summary , returns the number of candidates in that have equal or lower quality than , including itself. For example, if is the highest-quality candidate in , then .
can be obtained from human evaluators, automatic metrics (e.g. ROUGE) or heuristics measuring the quality of outputs. We denote the ground-truth ranking function onby .
Given the above definition of summarisation, we define a summariser agent as a tuple , where is an optimisation model for finding the (near-)optimal summary with respect to . In the cross-input paradigm, the RL agent learns a policy that solves the optimisation problem for any at training time. In RELIS, instead, an agent learns a ranking at training time so that is as “close” as possible to (“close” will be formally defined in §3). At test time, for each , RELIS formulates as a Markov Decision Process (MDP) and learns an RL policy specifically for .
2.2 Learning to Rank (L2R)
L2R algorithms learn to reconstruct the ranking over objects from an oracle [Li2011]. L2R induces the approximated ranking by learning a utility function from the oracle, such that if and only if . L2R can learn from different types of oracles, including pointwise oracles that provide point-based scores for objects, pairwise oracles that provide preferences over pairs of objects, and listwise oracles that provide certain metrics (e.g. accuracy) for candidate rankings. Here, we focus on pointwise and pairwise oracles as humans reliably provide such judgements for short texts [Kreutzer et al.2018]. Function can be learnt by any function approximation technique, e.g., neural networks.
In pointwise L2R, for every , we draw sample summaries from using some sampling strategy, e.g., random sampling without replacement. Then, we query their values from the oracle and use a regression algorithm to minimise the averaged mean squared error (MSE) between and .
Formally, the loss function is
In pairwise L2R, for every we sample pairs of summaries from and then query the oracle about their preferences. We denote the collected preferences for input by , where are the candidate outputs presented to the oracle in the iteration; equals if the oracle prefers over , and equals otherwise. Different loss functions can be used to learn from the preferences in . First, we consider the cross-entropy loss:
where . An alternative is the margin ranking (a.k.a. pairwise hinge) loss:
where if is preferred over , and otherwise. Additionally, we consider an improved margin ranking loss proposed by DBLP:conf/ecir/AgarwalC10 [DBLP:conf/ecir/AgarwalC10], which gives large penalties to mis-ranked pairs with wide margins:
Note that the improved margin ranking loss is a mix of pointwise and pairwise L2R as it requires both and .
2.3 Reinforcement Learning (RL)
RL amounts to algorithms that search for optimal solutions in Markov Decision Processes (MDPs). We formulate the optimisation problems for NLG tasks as episodic MDP [Ryang and Abekawa2012, Rioux et al.2014]. An episodic MDP is a tuple , where is a set of states, is a set of actions, is the transition function with giving the next state after performing action in state , is the reward function with giving the immediate reward for performing action in state and is a set of terminal states that mark the end of an episode. Furthermore, to formulate the delayed rewards in summarisation, we let if , so as to ensure that non-zero rewards only appear in the last step of each episode.
In extractive summarisation, the components in are defined as follows. is the set of all possible states, i.e. permutations of sentences from . Note that is a subset of because only includes the summaries complying with the length requirement, but includes all possible extractive summaries. Two types of action constitute : add a new sentence from the input document cluster to the current draft summary and terminate the generation. includes all the over-length summaries and an absorbing state . If action is terminate, regardless of the current state . The reward function returns if action is terminate, a negative reward if the current state is an over-length summary and 0 otherwise. We denote this MDP by to highlight that it uses as rewards.
A policy defines how actions are selected:
is the probability of selecting actionin state . In the context of summarisation, we slightly abuse the notation by letting denote the probability of policy generating given input . We say a policy is proper if , i.e. does not generate illegal outputs. Then, the expected reward of performing proper policy is:
The goal of RL is to find the optimal policy that has the highest expected reward: . Note that
is a probability distribution that assigns non-zero probabilities only to the optimal outputs for, i.e. only if . Hence .
3 Proof of Convergence for RELIS
In this section, we prove that if the errors in both L2R and RL are bounded, the error of the final output of RELIS is a linear polynomial of the errors from the two steps. First, we define convergent L2R, which can learn an approximated ranking “close” to the ground-truth ranking .
Let be the utility function learnt by an L2R algorithm. For , let be the ranking derived from on . The L2R algorithm is -convergent with respect to if there exists a non-negative integer such that
Some L2R algorithms are -convergent under realistic conditions. For example, the quick-sort based L2R algorithm proposed by DBLP:conf/icml/MaystreG17 [DBLP:conf/icml/MaystreG17] is -convergent with respect to pairwise oracles, when it obtains a sufficiently large number of preferences. Next, we define convergent RL algorithms that guarantee to learn near-optimal policies.
Let denote the episodic MDP for an NLG task given input , which uses a ranking function over as its reward function. An RL algorithm is -convergent for if there exists such that
where is the optimal policy for and is the policy of the RL algorithm after episodes of learning. 222Since is the optimal policy for , is always higher than the expected reward of any other policy (cf. Eq. 5).
Recall that for an input from the test set , our goal is to find the optimal policy for the NLG tuple (see §2). The theorem below shows that if RELIS uses convergent L2R and RL, its output is near-optimal, i.e. the error between the RELIS output and the optimal output is bounded.
For a test input , we denote the optimal NLG agent for by , and its RELIS approximation by , where is the approximation of learnt by an -convergent L2R algorithm. Let be the optimal policy for and the proper policy for learnt by a -convergent RL algorithm for episodes. Then, as approaches positive infinity, .
We denote the optimal policy for as . Hence from Eq. (7) we have:
For any , from Eq. (6) we have
4 Experimental Setup
Task and datasets.
We evaluate RELIS for extractive multi-document summarisation on three benchmark datasets from the Document Understanding Conferences (DUC)333https://duc.nist.gov/ described in Table 1. Each dataset contains a set of document clusters. For each cluster, the target is to create a summary with at most 100 words. Each cluster is accompanied with several human-generated summaries, denoted as , that we use for training and evaluation. To decide the best parameters, we perform 10-fold cross validation on DUC’01.
Ground-truth reward oracle.
For , we use a ROUGE-based to induce the ground-truth . We measure the quality of a summary by , where and denote the average ROUGE-1 and ROUGE-2 recall metrics, respectively. and are arguably the most widely used metrics to approximate human evaluation of summary quality. The scores for optimal extractive summaries are usually half of their scores [Gao et al.2018], so we multiply by to balance the impact of and . Next, we describe how we approximate the ground-truth reward.
|Dataset||# Cluster||# Doc||# Sent/Cluster|
4.1 Approximated Reward Oracle
Recall that the goal of L2R is to learn the utility function using pointwise or pairwise preferences (see §2). In pointwise L2R, we sample different summaries for each document cluster in by randomly selecting sentences from until the length limit is reached. Using larger does not significantly increase the quality of but increases the training time. We use the ground-truth ranking over the samples to learn the utility function by minimising the MSE loss from Eq. (1). In pairwise L2R, we randomly draw of the possible pairs of the sampled summaries to compute by minimising , or from Eq. (2) – (4). Preliminary results suggest that increasing to does not benefit the performance but significantly slows down the training, while decreasing it to significantly harms the quality of .
We use a linear model to approximate the utility function, i.e. , where encodes for input as a vector by concatenating the following features:
as a vector by concatenating the following features:
The negative Jensen-Shannon divergence between unigrams, bigrams and named entities in and .
The summary quality evaluation heuristics proposed by rioux2014emnlp [rioux2014emnlp].
TFIDF values averaged over all tokens and named entities in [Peyrard and Gurevych2018].
The average number of document clusters a word or named entity appears in.
Rate of named entities from appearing in .
The percentage of tokens in that belong to some named entities in .
The redundancy feature proposed by DBLP:conf/naacl/PeyrardG18 [DBLP:conf/naacl/PeyrardG18], applied to unigrams, bigrams and named entities.
We additionally use two Convolutional Neural Network (CNN) architectures to generate auto-encoded features: StdCNN[Kim2014] and PriorSum [Cao et al.2015]. To train these auto-encoders, we follow DBLP:conf/acl/CaoWLLZW15 [DBLP:conf/acl/CaoWLLZW15] and feed only the embeddings of the words of into these models. We add a linear layer as the last layer to map the output vector of these models to a value. Full settings of these models are in the supplementary material. For each , we measure the quality of the approximated ranking by its ranking correlation to the ground-truth . We consider two ranking correlation metrics: Spearman’s and the Normalized Discounted Cumulative Gain (ndcg) on the top items. ndcg puts more emphasis on the top elements with logarithmic decay weighting . For both metrics, higher values indicate stronger correlations .
We train the utility function with the loss functions in Eq. (1)-(4) and we find that using different loss functions does not significantly444In all experiments, we preform statistical significant test by two-tailed -test and -value . change performance (see supplementary material). However, the cross-entropy loss consistently results in marginally better performance than the others, and hence we use it throughout our experiments. We find that under all examined settings, the CNN-based features underperform the other features. Using the CNN-based features together with other features also worsen the quality of . The reason is because both PriorSum and StdCNN only encode the summaries’ document-independent features [Cao et al.2015]. Encoding document-dependent features requires more sophisticated neural models [Wu and Hu2018, Narayan et al.2018] which require considerable time, data and parameter tuning. This undermines the benefits of RELIS. We leave the research for efficient reward representation learning for future work.
4.2 RELIS Setup
We test RELIS on all three DUC datasets as follows. In line with DBLP:conf/aaai/CaoLLW17 [DBLP:conf/aaai/CaoLLW17] and DBLP:journals/tois/RenCRWNMR18 [DBLP:journals/tois/RenCRWNMR18], we split train and test data in the “leave-one-out” manner so that documents from two datasets are used as the training set, , and documents from the rest as the test set . In each run in the “leave-one-out” experiments, we randomly select 30% data from the training set as the dev set, and select the model with the best performance on the dev set. We use Adam with initial learning rate
. The number of epochs isand batch size is . As for RL in RELIS, we use the same temporal difference algorithm as rioux2014emnlp [rioux2014emnlp]. Full details of our parameter selection and results of using different loss functions are in the supplementary material.
Table 2 compares the quality of our with other widely used rewards for input-specific RL (see §4). has significantly higher correlation to the ground-truth ranking compared with all other approaches, confirming that our proposed L2R method yields a superior reward oracle.
In Table 3, we compare RELIS with non-RL-based and RL-based summarisation systems. For non-RL-based systems, we report
ICSI [Gillick and
maximising the bigram overlap of summary and input using integer linear programming to estimate
maximising the bigram overlap of summary and input using integer linear programming, PriorSum [Cao et al.2015] learning sentence quality with CNNs, TCSum [Cao et al.2017] employing text classification of the input documents, the variant of TCSum without the text classification pre-training (TCSum) and SRSum [Ren et al.2018], which learns sentence relations with both word- and sentence-level attentive neural networks
For RL-based systems, we re-implement REAPER [Rioux et al.2014] as an input-specific RL, and DeepTD as a cross-input RL. DeepTD is adapted from the DQN-based RL summariser [Yao et al.2018] and is trained by taking as the rewards (see §4). It uses InferSent to represent summaries. To improve the efficiency and performance of DeepTD, we use memory replay and periodic update. Further details and the algorithm of DeepTD are in the supplementary material.
RELIS significantly outperforms the other RL-based systems. Note that RELIS and REAPER use the identical RL algorithm for input-specific policy learning; hence the improvement of RELIS is due to the higher quality of the L2R-learnt reward . RELIS outperforms DeepTD because training cross-input policies requires much more data than available in the DUC datasets. At the same time, RELIS performs on par with neural-based TCSum and SRSum, while it requires significantly less data and time to train, as shown next.
We run RELIS, SRSum, DeepTD and REAPER on the same workstation with a 4-core CPU, 8 GB memory and no GPUs. Table 4 compares their average training and test time for each document cluster. RELIS reduces the training time of SRSum by two orders of magnitude. At test time, RELIS takes reasonable time to train the input-specific policy, and we believe that it can be further reduced by using more efficient RL algorithms and employing techniques like memory replay or reward shaping.
Unlike TCSum, RELIS requires no additional training data: TCSum uses 1.8 million news articles from New York Times and their category annotations (e.g. Health, Politics, Business) for training. It is worth noting that without using such a massive extra data for the text classification step, the performance of TCSum significantly drops (see TCSum in Table 3). The training time of TCSum is unlikely to be shorter than RELIS since TCSum requires to train
a CNN-based text classifierbefore training a CNN-based sentence selector.
|Training||810 s||1,560 s||N/A||2 s|
|Test||7 s||4 s||31 s||34 s|
To summarise, RELIS significantly outperforms RL-based models, and it yields competitive performance compared with the state-of-the-art neural summarisers, with the benefit of needing less training time and data.
6 Discussion & Related Work
RELIS is proposed as a RL-based summarisation paradigm, but it can be applied to other NLG tasks where RL is widely used, e.g., translation, sentence simplification and dialogue generation. For example, to apply RELIS to translation, we just let (see §2) be the set of texts in the source language and let be the set of all possible translations in the target language for input ; and the error bound of RELIS (Theorem 1) still holds as it is indifferent to the contents in and . Hence, in this section, we discuss the related work in the context of NLG in general.
recently receives increasing interests from the machine learning community[Ibarz et al.2018, Zheng et al.2018], but it is largely overlooked in NLG until now. Unlike classic RL applications, e.g. robotics and games, where rewards are provided or easy to design, NLG tasks lack strong metrics to measure the quality of the output. Well-known rewards, e.g. ROUGE, are criticised for their low correlation with human evaluations [Chaganty et al.2018]. Recent work suggests that improving the reward function boosts the performance of RL-based NLG [Kryscinski et al.2018].
Besides using metrics such as BLEU and ROUGE as rewards to train RL-based NLG, novel rewards have been designed. DBLP:conf/emnlp/KryscinskiPXS18 [DBLP:conf/emnlp/KryscinskiPXS18] propose a new reward function for encouraging RL-based summarisers to prefer novel words in abstractive summaries. For sentence simplification, DBLP:conf/emnlp/ZhangL17 [DBLP:conf/emnlp/ZhangL17] propose a reward function measuring the simplicity, relevance and grammatical correctness of candidate outputs. For machine translation, DBLP:conf/emnlp/NguyenDB17 [DBLP:conf/emnlp/NguyenDB17] propose a simulated user, which simulates human’s ratings on translations, and they use the simulated user to provide rewards for an RL-based translator. However, all rewards discussed above require reference outputs, unlike our learnt reward function which can be used to provide rewards at test time when no reference outputs are available.
DBLP:conf/aaai/WuH18 [DBLP:conf/aaai/WuH18] and DBLP:conf/naacl/BosselutCHGHC18 [DBLP:conf/naacl/BosselutCHGHC18] recently propose to use a large volume of unlabelled data to learn a scorer measuring the discourse coherence degree of sentences, and use the scorer as rewards to train cross-input RL. We go beyond their work by proving the soundness of the combination of reward learning and RL, and training input-specific instead of cross-input policies so as to reduce training time and data.
RL-based interactive NLG methods
elicit human feedback as rewards.
DBLP:conf/acl/RiezlerKU18 [DBLP:conf/acl/RiezlerKU18] elicit
pointwise and pairwise feedback on candidate
translations to train a cross-input policy.
DBLP:conf/emnlp/GaoMG18 [DBLP:conf/emnlp/GaoMG18] propose using active learning
using active learningto select appropriate candidate summary pairs and acquire human preferences to improve a heuristics-based reward. However, these methods require much feedback to bring satisfactory results, e.g., at least 50 summary pairs to yield significant improvements [Gao et al.2018]. RELIS can be used as a pre-training stage for interactive methods, as RELIS first learns a high-quality cross-input reward function and then uses the interactive NLG techniques to elicit a small amount of feedback to fine-tune a user- and input-specific reward function, so at to generate higher-quality and personalised results.
We propose a novel RL paradigm called RELIS, which learns a reward function from a reward oracle with learning-to-rank (L2R) algorithms at training time, and then uses the learnt reward to train input-specific RL policies at test time. Compared with the widely employed cross-input RL-based summarisation approaches, RELIS avoids the expensive learning of cross-input policies but, instead, efficiently performs L2R and input-specific RL learning. Moreover, RELIS avoids the arduous reward design required in input-specific RL-based summarisation approaches. We prove that, with proper L2R and RL algorithms, RELIS guarantees to produce near-optimal outputs. Our experiments show that even with linear L2R and standard RL algorithms, RELIS yields performance on par with the state of the art while only requiring a small fraction of data and time to train. Our work lays the theoretical foundation for reward learning in NLG, and we hope it will encourage further research in this direction.
This work has been supported by the German Research Foundation (DFG), as part of the QA-EduInf project (GU 798/18-1 and RI 803/12-1) and through the German-Israeli Project Cooperation (DIP, DA 1600/1-1 and GU 798/17-1).
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