Ranking Sentences for Extractive Summarization with Reinforcement Learning
Summary
The paper addresses single-document extractive summarization. The authors argue that standard training, which uses cross-entropy against sentence-level labels, fits the task poorly. Those labels are derived heuristically from abstractive reference summaries. The objective also never sees ROUGE, which scores the summary as a whole. As a result, such models tend to produce long, redundant extracts. The authors instead treat extraction as sentence ranking. Their model, REFRESH, has three parts: a convolutional sentence encoder, an LSTM document encoder, and an LSTM sentence extractor that scores each sentence's suitability for the summary. During training, the top-scored sentences form a candidate summary, which is compared with the gold highlights. The resulting ROUGE reward updates the network through REINFORCE, combined with the maximum-likelihood signal. Sentences that often appear in high-scoring extracts therefore get pushed up the ranking. On the CNN and DailyMail highlights datasets, REFRESH beats strong extractive and abstractive baselines on ROUGE and in two human studies. This shows that a globally trained extractor can outperform contemporary abstractive models.
Key points
- Data: news articles with highlight summaries from CNN and DailyMail, as introduced by Hermann et al. (2015). Code and data are released at github.com/shashiongithub/Refresh.
- Architecture:
- Sentence encoder: temporal narrow convolution with several kernel widths, followed by max-pooling over time.
- Document encoder: LSTM that reads sentences in reverse order, so the lead sentences are emphasized.
- Extractor: LSTM with a softmax layer; its confidence scores rank the sentences.
- Labeling: the authors contrast two ways of deriving labels.
- Individual oracle: each sentence is labeled separately by its match to the gold summary, which tends to yield too many positives.
- Collective oracle: the sentence subset with the highest joint ROUGE is labeled positive.
- Question-answering evaluation: readers answered 66.34% of questions correctly from REFRESH summaries, versus 36.33% for LEAD and 28.73% for See et al. (2017).
- Ranking evaluation: REFRESH was ranked second, and its difference from GOLD was not statistically significant (one-way ANOVA with post-hoc Tukey HSD, p < 0.01).
Interesting ideas
- I1. Reward the assembled output, not each label. The model is trained by scoring the summary actually built from its top-ranked sentences against the metric used at test time. This transfers to any top-k selection task, such as retrieval or keyphrase picking, whose evaluation measure is non-differentiable and set-level.
- I2. Measure informativeness with questions. Instead of relying only on n-gram overlap, the authors check how many questions about the article readers can answer from a summary. This makes differences between systems far sharper and better reflects what users actually need.
- I3. Separate preference from content. Human rankings placed See et al. above LEAD, yet LEAD carried more answerable content. The authors attribute this to readers favoring shorter texts (61.6 vs 105.7 words on average). It is a useful warning to control for length, or measure it, in any human evaluation.
- I4. Build oracle labels for sets, not single items. Selecting the jointly best-scoring subset gives cleaner supervision than labeling sentences one by one. The same idea applies whenever silver labels must be extracted from abstractive references.
- I5. Feed the document in reverse order. Reading sentences backwards into the encoder keeps the opening of the document salient in its representation. This is a cheap trick for any genre where early content matters most.
Cited by
- Re-evaluating Evaluation in Text Summarization
- Fast Abstractive Summarization with Reinforce-Selected Sentence Rewriting
- GSum: A General Framework for Guided Neural Abstractive Summarization
- CTRLsum: Towards Generic Controllable Text Summarization
- Neural Text Summarization: A Critical Evaluation
- Hierarchical Transformers for Multi-Document Summarization
- Encode, Tag, Realize: High-Precision Text Editing
- Leveraging Pre-trained Checkpoints for Sequence Generation Tasks
- Answers Unite! Unsupervised Metrics for Reinforced Summarization Models
- MLSUM: The Multilingual Summarization Corpus
- Heterogeneous Graph Neural Networks for Extractive Document Summarization
- Discourse-Aware Neural Extractive Text Summarization
- BARTScore: Evaluating Generated Text as Text Generation
- HIBERT: Document Level Pre-training of Hierarchical Bidirectional Transformers for Document Summarization
- Extractive Summarization as Text Matching
Cite
@inproceedings{narayan2018ranking,
title = {{Ranking Sentences for Extractive Summarization with Reinforcement Learning}},
author = {Narayan, Shashi and Cohen, Shay B. and Lapata, Mirella},
booktitle = {Proceedings of the 2018 Conference of the North American Chapter of the Association for Computational Linguistics: Human Language Technologies, Volume 1 (Long Papers)},
year = {2018},
doi = {10.18653/v1/n18-1158},
url = {https://doi.org/10.18653/v1/n18-1158}
}References
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