Neural Network Models for Paraphrase Identification, Semantic Textual Similarity, Natural Language Inference, and Question Answering

06/12/2018
by   Wuwei Lan, et al.
The Ohio State University
0

In this paper, we analyze several neural network designs (and their variations) for sentence pair modeling and compare their performance extensively across eight datasets, including paraphrase identification, semantic textual similarity, natural language inference, and question answering tasks. Although most of these models have claimed state-of-the-art performance, the original papers often reported on only one or two selected datasets. We provide a systematic study and show that (i) encoding contextual information by LSTM and inter-sentence interactions are critical, (ii) Tree-LSTM does not help as much as previously claimed but surprisingly improves performance on Twitter datasets, (iii) the Enhanced Sequential Inference Model is the best so far for larger datasets, while the Pairwise Word Interaction Model achieves the best performance when less data is available. We release our implementations as an open-source toolkit.

READ FULL TEXT VIEW PDF

Authors

page 1

page 2

page 3

page 4

04/20/2018

Learning Semantic Textual Similarity from Conversations

We present a novel approach to learn representations for sentence-level ...
10/29/2019

Sentence Embeddings for Russian NLU

We investigate the performance of sentence embeddings models on several ...
05/21/2018

Character-based Neural Networks for Sentence Pair Modeling

Sentence pair modeling is critical for many NLP tasks, such as paraphras...
08/26/2019

Ensemble approach for natural language question answering problem

Machine comprehension, answering a question depending on a given context...
05/24/2021

Augmenting Modelers with Semantic Autocompletion of Processes

Business process modelers need to have expertise and knowledge of the do...
11/07/2019

Explicit Pairwise Word Interaction Modeling Improves Pretrained Transformers for English Semantic Similarity Tasks

In English semantic similarity tasks, classic word embedding-based appro...
09/27/2018

A Qualitative Comparison of CoQA, SQuAD 2.0 and QuAC

In this work, we compare three datasets which build on the paradigm defi...
This week in AI

Get the week's most popular data science and artificial intelligence research sent straight to your inbox every Saturday.

1 Introduction

This work is licensed under a Creative Commons Attribution 4.0 International License. License details: http://creativecommons.org/licenses/by/4.0/.

Sentence pair modeling is a fundamental technique underlying many NLP tasks, including the following:

  • Semantic Textual Similarity (STS), which measures the degree of equivalence in the underlying semantics of paired snippets of text [Agirre et al.2016].

  • Paraphrase Identification (PI), which identifies whether two sentences express the same meaning [Dolan and Brockett2005, Xu et al.2014, Xu et al.2015].

  • Natural Language Inference (NLI), also known as recognizing textual entailment (RTE), which concerns whether a hypothesis can be inferred from a premise, requiring understanding of the semantic similarity between the hypothesis and the premise [Dagan et al.2006, Bowman et al.2015].

  • Question Answering (QA), which can be approximated as ranking candidate answer sentences or phrases based on their similarity to the original question [Yang et al.2015].

  • Machine Comprehension (MC), which requires sentence matching between a passage and a question, pointing out the text region that contains the answer. [Rajpurkar et al.2016].

Traditionally, researchers had to develop different methods specific for each task. Now neural networks can perform all the above tasks with the same architecture by training end to end. Various neural models [He and Lin2016, Chen et al.2017, Parikh et al.2016, Wieting et al.2016, Tomar et al.2017, Wang et al.2017, Shen et al.2017a, Yin et al.2016]

have declared state-of-the-art results for sentence pair modeling tasks; however, they were carefully designed and evaluated on selected (often one or two) datasets that can demonstrate the superiority of the model. The research questions are as follows: Do they perform well on other tasks and datasets? How much performance gain is due to certain system design choices and hyperparameter optimizations?

To answer these questions and better understand different network designs, we systematically analyze and compare the state-of-the-art neural models across multiple tasks and multiple domains. Namely, we implement five models and their variations on the same PyTorch platform: InferSent model

[Conneau et al.2017], Shortcut-stacked Sentence Encoder Model [Nie and Bansal2017], Pairwise Word Interaction Model [He and Lin2016]

, Decomposable Attention Model

[Parikh et al.2016], and Enhanced Sequential Inference Model [Chen et al.2017]. They are representative of the two most common approaches: sentence encoding models

that learn vector representations of individual sentences and then calculate the semantic relationship between sentences based on vector distance and

sentence pair interaction models that use some sorts of word alignment mechanisms (e.g., attention) then aggregate inter-sentence interactions. We focus on identifying important network designs and present a series of findings with quantitative measurements and in-depth analyses, including (i) incorporating inter-sentence interactions is critical; (ii) Tree-LSTM does not help as much as previously claimed but surprisingly improves performance on Twitter data; (iii) Enhanced Sequential Inference Model has the most consistent high performance for larger datasets, while Pairwise Word Interaction Model performs better on smaller datasets and Shortcut-Stacked Sentence Encoder Model is the best performaning model on the Quora corpus. We release our implementations as a toolkit to the research community.111The code is available on the authors’ homepages and GitHub: https://github.com/lanwuwei/SPM_toolkit

2 General Framework for Sentence Pair Modeling

Various neural networks have been proposed for sentence pair modeling, all of which fall into two types of approaches. The sentence encoding approach encodes each sentence into a fixed-length vector and then computes sentence similarity directly. The model of this type has advantages in the simplicity of the network design and generalization to other NLP tasks. The sentence pair interaction approach takes word alignment and interactions between the sentence pair into account and often show better performance when trained on in-domain data. Here we outline the two types of neural networks under the same general framework:

  • The Input Embedding Layer takes vector representations of words as input, where pretrained word embeddings are most commonly used, e.g. GloVe [Pennington et al.2014] or Word2vec [Mikolov et al.2013]. Some work used embeddings specially trained on phrase or sentence pairs that are paraphrases [Wieting and Gimpel2017, Tomar et al.2017]; some used subword embeddings, which showed improvement on social media data [Lan and Xu2018].

  • The Context Encoding Layer incorporates word context and sequence order into modeling for better vector representation. This layer often uses CNN [He et al.2015], LSTM [Chen et al.2017], recursive neural network [Socher et al.2011], or highway network [Gong et al.2017]. The sentence encoding type of model will stop at this step, and directly use the encoded vectors to compute the semantic similarity through vector distances and/or the output classification layer.

  • The Interaction and Attention Layer

    calculates word pair (or n-gram pair) interactions using the outputs of the encoding layer. This is the key component for the interaction-aggregation type of model. In the PWIM model

    [He and Lin2016]

    , the interactions are calculated by cosine similarity, Euclidean distance, and the dot product of the vectors. Various models put different weights on different interactions, primarily simulating the word alignment between two sentences. The alignment information is useful for sentence pair modeling because the semantic relation between two sentences depends largely on the relations of aligned chunks as shown in the SemEval-2016 task of interpretable semantic textual similarity

    [Agirre et al.2016].

  • The Output Classification Layer

    adapts CNN or MLP to extract semantic-level features on the attentive alignment and applies softmax function to predict probability for each class.

3 Representative Models for Sentence Pair Modeling

Table 1 gives a summary of typical models for sentence pair modeling in recent years. In particular, we investigate five models in depth: two are representative of the sentence encoding type of model, and three are representative of the interaction-aggregation type of model. These models have reported state-or-the-art results with varied architecture design (this section) and implementation details (Section 4.2).

Models Sentence Interaction and Aggregation and
Encoder Attention Classification
[Shen et al.2017b] Directional self-attention network - MLP
[Choi et al.2017] Gumbel Tree-LSTM - MLP
[Wieting and Gimpel2017] Gated recurrent average network - MLP
SSE [Nie and Bansal2017] Shortcut-stacked BiLSTM - MLP
[He et al.2015] CNN multi-perspective matching pooling + MLP
[Rocktäschel et al.2016] LSTM word-by-word neural attention MLP
[Liu et al.2016] LSTM coupled LSTMs dynamic pooling + MLP
[Yin et al.2016] CNN attention matrix logistic regression
DecAtt [Parikh et al.2016] - dot product + soft alignment summation + MLP
PWIM [He and Lin2016] BiLSTM cosine, Euclidean, dot product + hard alignment CNN + MLP
[Wang and Jiang2017] LSTM encodes both context and attention word-by-word neural attention MLP
ESIM [Chen et al.2017] BiLSTM (Tree-LSTM) before and after attention dot product + soft alignment

average and max pooling + MLP

[Wang et al.2017] BiLSTM multi-perspective matching BiLSTM + MLP
[Shen et al.2017a] BiLSTM + intra-attention soft alignment + orthogonal decomposition MLP
[Ghaeini et al.2018] dependent reading BiLSTM dot product + soft alignment average and max pooling+MLP
Table 1: Summary of representative neural models for sentence pair modeling. The upper half contains sentence encoding models, and the lower half contains sentence pair interaction models.
(a) InferSent
(b) SSE
(c) Classification Layer
Figure 1: Sentence encoding models focus on learning vector representations of individual sentences and then calculate the semantic relationship between sentences based on vector distance.

3.1 The Bi-LSTM Max-pooling Network (InferSent)

We choose the simple Bi-LSTM max-pooling network from InferSent [Conneau et al.2017]:

(1)
(2)

where

represents the concatenation of hidden states in both directons. It has shown better transfer learning capabilities than several other sentence embedding models, including SkipThought

[Kiros et al.2015] and FastSent [Hill et al.2016], when trained on the natural language inference datasets.

3.2 The Shortcut-Stacked Sentence Encoder Model (SSE)

The Shortcut-Stacked Sentence Encoder model [Nie and Bansal2017] is a sentence-based embedding model, which enhances multi-layer Bi-LSTM with skip connection to avoid training error accumulation, and calculates each layer as follows:

(3)
(4)
(5)

where is the input of the th Bi-LSTM layer at time step , which is the combination of outputs from all previous layers, represents the hidden state of the th Bi-LSTM layer in both directions. The final sentence embedding is the row-based max pooling over the output of the last Bi-LSTM layer, where denotes the number of words within a sentence and is the number of Bi-LSTM layers ( in SSE).

3.3 The Pairwise Word Interaction Model (PWIM)

In the Pairwise Word Interaction model [He and Lin2016], each word vector is encoded with context through forward and backward LSTMs: and . For every word pair across sentences, the model directly calculates word pair interactions using cosine similarity, Euclidean distance, and dot product over the outputs of the encoding layer:

(6)

The above equation not only applies to forward hidden state and backward hidden state , but also to the concatenation and summation

, resulting in a tensor

after padding one extra bias term. A “hard” attention is applied to the interaction tensor to build word alignment: selecting the most related word pairs and increasing the corresponding weights by 10 times. Then a 19-layer deep CNN is applied to aggregate the word interaction features for final classification.

3.4 The Decomposable Attention Model (DecAtt)

The Decomposable Attention model [Parikh et al.2016] is one of the earliest models to introduce attention-based alignment for sentence pair modeling, and it achieved state-of-the-art results on the SNLI dataset with about an order of magnitude fewer parameters than other models (see more in Table 5) without relying on word order information. It computes the word pair interaction between and (from input sentences and , each with and words, respectively) as , where is a feedforward network; then alignment is determined as follows:

(7)

where is the soft alignment between and subphrases in sentence , and vice versa for . The aligned phrases are fed into another feedforward network : and to generate sets and , which are aggregated by summation and then concatenated together for classification.

3.5 The Enhanced Sequential Inference Model (ESIM)

The Enhanced Sequential Inference Model [Chen et al.2017] is closely related to the DecAtt model, but it differs in a few aspects. First, Chen et al. Chen-Qian:2017:ACL demonstrated that using Bi-LSTM to encode sequential contexts is important for performance improvement. They used the concatenation of both directions as in the PWIM model. The word alignment and between and are calculated the same way as in DecAtt. Second, they showed the competitive performance of recursive architecture with constituency parsing, which complements with sequential LSTM. The feedforward function in DecAtt is replaced with Tree-LSTM:

(8)
(9)

Third, instead of using summation in aggregation, ESIM adapts the average and max pooling and concatenation

before passing through multi-layer perceptron (MLP) for classification:

(10)
(a) PWIM
(b) ESIM (DecAtt is similar and simpler.)
Figure 2: Sentence pair interaction models use different word alignment mechanisms before aggregation.

4 Experiments and Analysis

4.1 Datasets

We conducted sentence pair modeling experiments on eight popular datasets: two NLI datasets, three PI datasets, one STS dataset and two QA datasets. Table 2 gives a comparison of these datasets:

Dataset Size Example and Label
SNLI train 550,152 : Two men on bicycles competing in a race. entailment
dev 10,000 : Men are riding bicycles on the street. neutral
test 10,000 contradict
Multi-NLI train 392,703 : The Old One always comforted Ca’daan, except today. entailment
dev 20,000 : Ca’daan knew the Old One very well. neutral
test 20,000 contradict
Quora train 384,348 : What should I do to avoid sleeping in class? paraphrase
dev 10,000 : How do I not sleep in a boring class? non-paraphrase
test 10,000
Twitter-URL train 42,200 : Letter warned Wells Fargo of “widespread” fraud in 2007. paraphrase
dev - : Letters suggest Wells Fargo scandal started earlier. non-paraphrase
test 9,324
PIT-2015 train 11,530 : Ezekiel Ansah w the 3D shades Popped out lens paraphrase
dev 4,142 : Ezekiel Ansah was wearing lens less 3D glasses non-paraphrase
test 838
STS-2014 train 7,592 : Then perhaps we could have avoided a catastrophe. score
dev - : Then we might have been able to avoid a disaster.
test 3,750
WikiQA train 8,672 : How much is 1 tablespoon of water? true
dev 1,130 : In Australia one tablespoon (measurement unit) is 20 mL. false
test 2,351
TrecQA train 53,417 : Who was Lincoln’s Secretary of State? true
dev 1,148 : William Seward false
test 1,517
Table 2: Basic statistics and examples of different datasets for sentence pair modeling tasks.
  • SNLI [Bowman et al.2015] contains 570k hypotheses written by crowdsourcing workers given the premises. It focuses on three semantic relations: the premise entails the hypothesis (entailment), they contradict each other (contradiction), or they are unrelated (neutral).

  • Multi-NLI [Williams et al.2017] extends the SNLI corpus to multiple genres of written and spoken texts with 433k sentence pairs.

  • Quora [Iyer et al.2017] contains 400k question pairs collected from the Quora website. This dataset has balanced positive and negative labels indicating whether the questions are duplicated or not.

  • Twitter-URL [Lan et al.2017] includes 50k sentence pairs collected from tweets that share the same URL of news articles. This dataset contains both formal and informal language.

  • PIT-2015 [Xu et al.2015] comes from SemEval-2015 and was collected from tweets under the same trending topic. It contains naturally occurred (i.e. written by independent Twitter users spontaneously) paraphrases and non-paraphrases with varied topics and language styles.

  • STS-2014 [Agirre et al.2014] is from SemEval-2014, constructed from image descriptions, news headlines, tweet news, discussion forums, and OntoNotes [Hovy et al.2006].

  • WikiQA [Yang et al.2015] is an open-domain question-answering dataset. Following He and Lin he-lin:2016:N16-1, questions without correct candidate answer sentences are excluded, and answer sentences are truncated to 40 tokens, resulting in 12k question-answer pairs for our experiments.

  • TrecQA [Wang et al.2007] is an answer selection task of 56k question-answer pairs and created in Text Retrieval Conferences (TREC). For both WikiQA and TrecQA datasets, the best answer is selected according to the semantic relatedness with the question.

4.2 Implementation Details

We implement all the models with the same PyTorch framework.222InferSent and SSE have open-source PyTorch implementations by the original authors, for which we reused part of the code.333Our code is available at: https://github.com/lanwuwei/SPM_toolkit Below, we summarize the implementation details that are key for reproducing results for each model:

  • SSE:

    This model can converge very fast, for example, 2 or 3 epochs for the SNLI dataset. We control the convergence speed by updating the learning rate for each epoch: specifically,

    , where is the initial learning rate and is the index of current epoch.

  • DecAtt:

    It is important to use gradient clipping for this model: for each gradient update, we check the L2 norm of all the gradient values, if it is greater than a threshold

    , we scale the gradient by a factor . Another useful procedure is to assemble batches of sentences with similar length.

  • ESIM: Similar but different from DecAtt, ESIM batches sentences with varied length and uses masks to filter out padding information. In order to batch the parse trees within Tree-LSTM recursion, we follow Bowman et al.’s bowman-EtAl:2016:P16-1 procedure that converts tree structures into the linear sequential structure of a shift reduce parser. Two additional masks are used for producing left and right children of a tree node.

  • PWIM: The cosine and Euclidean distances used in the word interaction layer have smaller values for similar vectors while dot products have larger values. The performance increases if we add a negative sign to make all the vector similarity measurements behave consistently.

4.3 Analysis

4.3.1 Re-implementation Results vs. Previously Reported Results

Table 3 and 4 show the results reported in the original papers and the replicated results with our implementation. We use accuracy, F1 score, Pearson’s , Mean Average Precision (MAP), and Mean Reciprocal Rank (MRR) for evaluation on different datasets following the literature. Our reproduced results are slightly lower than the original results by 0.5

1.5 points on accuracy. We suspect the following potential reasons: (i) less extensive hyperparameter tuning for each individual dataset; (ii) only one run with random seeding to report results; and (iii) use of different neural network toolkits: for example, the original ESIM model was implemented with Theano, and PWIM model was in Torch.

4.3.2 Effects of Model Components

Herein, we examine the main components that account for performance in sentence pair modeling.

How important is LSTM encoded context information for sentence pair modeling?
Regarding DecAtt, Parikh et al. parikh-EtAl:2016:EMNLP2016 mentioned that “intra-sentence attention is optional”; they can achieve competitive results without considering context information. However, not surprisingly, our experiments consistently show that encoding sequential context information with LSTM is critical. Compared to DecAtt, ESIM shows better performance on every dataset (see Table 4 and Figure 3). The main difference between ESIM and DecAtt that contributes to performance improvement, we found, is the use of Bi-LSTM and Tree-LSTM for sentence encoding, rather than the different choices of aggregation functions.

Why does Tree-LSTM help with Twitter data?
Chen et al. Chen-Qian:2017:ACL offered a simple combination (ESIM) by averaging the prediction probabilities of two ESIM variants that use sequential Bi-LSTM and Tree-LSTM respectively, and suggested “parsing information complements very well with ESIM and further improves the performance”. However, we found that adding Tree-LSTM only helps slightly or not at all for most datasets, but it helps noticably with the two Twitter paraphrase datasets. We hypothesize the reason is that these two datasets come from real-world tweets which often contain extraneous text fragments, in contrast to SNLI and other datasets that have sentences written by crowdsourcing workers. For example, the segment “ever wondered ,” in the sentence pair ever wondered , why your recorded #voice sounds weird to you? and why do our recorded voices sound so weird to us? introduces a disruptive context into the Bi-LSTM encoder, while Tree-LSTM can put it in a less important position after constituency parsing.

Model SNLI Multi-NLI Quora Twitter-URL PIT-2015 STS-2014 WikiQA TrecQA
Acc Acc_m/Acc_um Acc F1 F1 MAP/MRR MAP/MRR
InferSent 0.845 -/- - - - 0.700555This number was generated by InferSent traind on SNLI and Multi-NLI datasets. - -
SSE 0.860 0.746/0.736 - - - - - -
DecAtt 0.863 - 0.865333This number was reported in [Tomar et al.2017] by co-authors of DecAtt [Parikh et al.2016]. - - - - -
ESIM 0.878 - - - - - - -
ESIM 0.880 0.723/0.721444This number was reproduced by Williams et al. williams2017broad. - - - - - -
ESIM 0.886 - - - - - - -
PWIM - - - 0.749 0.667 0.767 0.709/0.723 0.759/0.822
Table 3: Reported results from original papers, which are mostly limited to a few datasets. For the Multi-NLI dataset, Acc_m represents testing accuracy for the matched genre and Acc_um for the unmatched genre.
Model SNLI Multi-NLI Quora Twitter-URL PIT-2015 STS-2014 WikiQA TrecQA
Acc Acc_m/Acc_um Acc F1 F1 MAP/MRR MAP/MRR
InferSent 0.846 0.705/0.703 0.866 0.746 0.451 0.715 0.287/0.287 0.521/0.559
SSE 0.855 0.740/0.734 0.878 0.650 0.422 0.378 0.624/0.638 0.628/0.670
DecAtt 0.856 0.719/0.713 0.845 0.652 0.430 0.317 0.603/0.619 0.660/0.712
ESIM 0.864 0.736/0.727 0.755 0.740 0.447 0.493 0.618/0.633 0.698/0.734
ESIM 0.870 0.752/0.738 0.850 0.748 0.520 0.602 0.652/0.664 0.771/0.795
ESIM 0.871 0.753/0.748 0.854 0.759 0.538 0.589 0.647/0.658 0.749/0.768
PWIM 0.822 0.722/0.716 0.834 0.761 0.656 0.743 0.706/0.723 0.739/0.795
Table 4: Replicated results with our reimplementation in PyTorch across multiple tasks and datasets. The best result in each dataset is denoted by a bold typeface, and the second best is denoted by an underline.

How important is attentive interaction for sentence pair modeling? Why does SSE excel on Quora?
Both ESIM and DecAtt (Eq. 7) calculate an attention-based soft alignment between a sentence pair, which was also proposed in [Rocktäschel et al.2016] and [Wang and Jiang2017] for sentence pair modeling, whereas PWIM utilizes a hard attention mechanism. Both attention strategies are critical for model performance. In PWIM model [He and Lin2016], we observed a 12 point performance drop after removing the hard attention, 03 point performance drop and 25% training time reduction after removing the 19-layer CNN aggregation. Likely without even the authors of SSE knowing, the SSE model performs extraordinarily well on the Quora corpus, perhaps because Quora contains many sentence pairs with less complicated inter-sentence interactions (e.g., many identical words in the two sentences) and incorrect ground truth labels (e.g., What is your biggest regret in life? and What’s the biggest regret you’ve had in life? are labeled as non-duplicate questions by mistake).

4.3.3 Learning Curves and Training Time

Figure 3 shows the learning curves. The DecAtt model converges quickly and performs well on large NLI datasets due to its design simplicity. PWIM is the slowest model (see time comparison in Table 5) but shows very strong performance on semantic similarity and paraphrase identification datasets. ESIM and SSE keep a good balance between training time and performance.

Figure 3: Training curves of ESIM, DecAtt, PWIM, SSE and InferSent models on eight datasets.
InferSent SSE DecAtt ESIM ESIM PWIM
Number of parameters 47M 140M 380K 4.3M 7.7M 2.2M
Avg epoch time (seconds) / sentence pair 0.005 0.032 0.0006 0.013 0.016 0.60
Ratio compared to DecAtt model 8 53 1 22 26 1000
Table 5: Average training time per sentence pair in the Twitter-URL dataset (similar time for other datasets).

4.3.4 Effects of Training Data Size

As shown in Figure 5, we experimented with different training sizes of the largest SNLI dataset. All the models show improved performance as we increase the training size. ESIM and SSE have very similar trends and clearly outperform PWIM on the SNLI dataset. DecAtt shows a performance jump when the training size exceeds a threshold.

4.3.5 Categorical Performance Comparison

We conducted an in-depth analysis of model performance on the Multi-domain NLI dataset based on different categories: text genre, sentence pair overlap, and sentence length. As shown in Table 6, all models have comparable performance between matched genre and unmatched genre. Sentence length and overlap turn out to be two important factors – the longer the sentences and the fewer tokens in common, the more challenging it is to determine their semantic relationship. These phenomena shared by the state-of-the-art systems reflect their similar design framework which is symmetric at processing both sentences in the pair, while question answering and natural language inference tasks are directional [Ghaeini et al.2018]. How to incorporate asymmetry into model design will be worth more exploration in future research.

Figure 4: Performance vs. training size (log scale in x-axis) on SNLI dataset.
Models Quora URL PIT train/test
trained on Quora on PIT
InferSent 0.866 0.528 0.394 0.451
SSE 0.878 0.681 0.594 0.422
DecAtt 0.845 0.649 0.497 0.430
ESIM 0.850 0.643 0.501 0.520
PWIM 0.835 0.601 0.518 0.656
trained on URL
InferSent 0.703 0.746 0.535 0.451
SSE 0.630 0.650 0.477 0.422
DecAtt 0.632 0.652 0.450 0.430
ESIM 0.641 0.748 0.511 0.520
PWIM 0.678 0.761 0.634 0.656
Figure 5: Transfer learning experiments for paraphrase identification task.
Category #Examples InferSent SSE DecAtt ESIM PWIM
Matched Fiction 1973 0.703 0.727 0.706 0.742 0.707
Genre Government 1945 0.753 0.746 0.743 0.790 0.751
Slate 1955 0.653 0.670 0.671 0.697 0.670
Telephone 1966 0.718 0.728 0.717 0.753 0.709
Travel 1976 0.705 0.701 0.733 0.752 0.714
Mismatched 9/11 1974 0.685 0.710 0.699 0.737 0.711
Genre Face-to-face 1974 0.713 0.729 0.720 0.761 0.710
Letters 1977 0.734 0.757 0.754 0.775 0.757
OUP 1961 0.698 0.715 0.719 0.759 0.710
Verbatim 1946 0.691 0.701 0.709 0.725 0.713
Overlap 60% 488 0.756 0.795 0.805 0.842 0.811
30% 60% 3225 0.740 0.751 0.745 0.769 0.743
30% 6102 0.685 0.689 0.691 0.727 0.682
Length 20 tokens 3730 0.692 0.676 0.685 0.731 0.694
1020 tokens 3673 0.712 0.725 0.721 0.753 0.720
10 tokens 2412 0.721 0.758 0.748 0.762 0.724
Table 6: Categorical performance (accuracy) on Multi-NLI dataset. Overlap is the percentage of shared tokens between two sentences. Length is calculated based on the number of tokens of the longer sentence.

4.3.6 Transfer Learning Experiments

In addition to the cross-domain study (Table 6), we conducted transfer learning experiments on three paraphrase identification datasets (Table 5). The most noteworthy phenomenon is that the SSE model performs better on Twitter-URL and PIT-2015 when trained on the large out-of-domain Quora data than the small in-domain training data. Two likely reasons are: (i) the SSE model with over 29 million parameters is data hungry and (ii) SSE model is a sentence encoding model, which generalizes better across domains/tasks than sentence pair interaction models. Sentence pair interaction models may encounter difficulties on Quora, which contains sentence pairs with the highest word overlap (51.5%) among all datasets and often causes the interaction patterns to focus on a few key words that differ. In contrast, the Twitter-URL dataset has the lowest overlap (23.0%) with a semantic relationship that is mainly based on the intention of the tweets.

5 Conclusion

We analyzed five different neural models (and their variations) for sentence pair modeling and conducted a series of experiments with eight representative datasets for different NLP tasks. We quantified the importance of the LSTM encoder and attentive alignment for inter-sentence interaction, as well as the transfer learning ability of sentence encoding based models. We showed that the SNLI corpus of over 550k sentence pairs cannot saturate the learning curve. We systematically compared the strengths and weaknesses of different network designs and provided insights for future work.

Acknowledgements

We thank Ohio Supercomputer Center [Center2012] for computing resources. This work was supported in part by NSF CRII award (RI-1755898) and DARPA through the ARO (W911NF-17-C-0095). The content of the information in this document does not necessarily reflect the position or the policy of the U.S. Government, and no official endorsement should be inferred.

References

  • [Agirre et al.2014] Eneko Agirre, Carmen Banea, Claire Cardie, Daniel Cer, Mona Diab, Aitor Gonzalez-Agirre, Weiwei Guo, Rada Mihalcea, German Rigau, and Janyce Wiebe. 2014. Semeval-2014 task 10: Multilingual semantic textual similarity. In Proceedings of the 8th International Workshop on Semantic Evaluation (SemEval 2014).
  • [Agirre et al.2016] Eneko Agirre, Aitor Gonzalez-Agirre, Inigo Lopez-Gazpio, Montse Maritxalar, German Rigau, and Larraitz Uria. 2016. Semeval-2016 task 2: Interpretable semantic textual similarity. In Proceedings of the 10th International Workshop on Semantic Evaluation (SemEval).
  • [Bowman et al.2015] Samuel R. Bowman, Gabor Angeli, Christopher Potts, and Christopher D. Manning. 2015. A large annotated corpus for learning natural language inference. In

    Proceedings of the 2015 Conference on Empirical Methods in Natural Language Processing (EMNLP)

    .
  • [Bowman et al.2016] Samuel R. Bowman, Jon Gauthier, Abhinav Rastogi, Raghav Gupta, Christopher D. Manning, and Christopher Potts. 2016. A fast unified model for parsing and sentence understanding. In Proceedings of the 54th Annual Meeting of the Association for Computational Linguistics (ACL).
  • [Center2012] Ohio Supercomputer Center. 2012. Oakley supercomputer. http://osc.edu/ark:/19495/hpc0cvqn.
  • [Chen et al.2017] Qian Chen, Xiaodan Zhu, Zhenhua Ling, Si Wei, Hui Jiang, and Diana Inkpen. 2017. Enhanced LSTM for natural language inference. In Proceedings of the 55th Annual Meeting of the Association for Computational Linguistics (ACL).
  • [Choi et al.2017] Jihun Choi, Kang Min Yoo, and Sang-goo Lee. 2017. Unsupervised learning of task-specific tree structures with tree-LSTMs. arXiv preprint arXiv:1707.02786.
  • [Conneau et al.2017] Alexis Conneau, Douwe Kiela, Holger Schwenk, Loïc Barrault, and Antoine Bordes. 2017. Supervised learning of universal sentence representations from natural language inference data. In Proceedings of the 2017 Conference on Empirical Methods in Natural Language Processing (EMNLP).
  • [Dagan et al.2006] Ido Dagan, Oren Glickman, and Bernardo Magnini. 2006. The PASCAL recognising textual entailment challenge. In

    Proceedings of the First International Conference on Machine Learning Challenges: Evaluating Predictive Uncertainty Visual Object Classification, and Recognizing Textual Entailment

    .
  • [Dolan and Brockett2005] William B Dolan and Chris Brockett. 2005. Automatically constructing a corpus of sentential paraphrases. In Proceedings of the Third International Workshop on Paraphrasing (IWP).
  • [Ghaeini et al.2018] Reza Ghaeini, Sadid A Hasan, Vivek Datla, Joey Liu, Kathy Lee, Ashequl Qadir, Yuan Ling, Aaditya Prakash, Xiaoli Z Fern, and Oladimeji Farri. 2018. DR-BiLSTM: Dependent reading bidirectional LSTM for natural language inference. In Proceedings of the 2018 Conference of the North American Chapter of the Association for Computational Linguistics: Human Language Technologies (NAACL-HLT).
  • [Gong et al.2017] Yichen Gong, Heng Luo, and Jian Zhang. 2017. Natural language inference over interaction space. arXiv preprint arXiv:1709.04348.
  • [He and Lin2016] Hua He and Jimmy Lin. 2016. Pairwise word interaction modeling with deep neural networks for semantic similarity measurement. In Proceedings of the 2016 Conference of the North American Chapter of the Association for Computational Linguistics: Human Language Technologies (NAACL-HLT).
  • [He et al.2015] Hua He, Kevin Gimpel, and Jimmy Lin. 2015.

    Multi-perspective sentence similarity modeling with convolutional neural networks.

    In Proceedings of the 2015 Conference on Empirical Methods in Natural Language Processing (EMNLP).
  • [Hill et al.2016] Felix Hill, Kyunghyun Cho, and Anna Korhonen. 2016.

    Learning distributed representations of sentences from unlabelled data.

    In Proceedings of the 2016 Conference of the North American Chapter of the Association for Computational Linguistics: Human Language Technologies (NAACL-HLT).
  • [Hovy et al.2006] Eduard Hovy, Mitchell Marcus, Martha Palmer, Lance Ramshaw, and Ralph Weischedel. 2006. Ontonotes: The 90% solution. In Proceedings of the Human Language Technology Conference of the North American Chapter of the ACL (NAACL).
  • [Iyer et al.2017] Shankar Iyer, Nikhil Dandekar, and Kornél Csernai. 2017. First Quora Dataset Release: Question Pairs. In https://data.quora.com/First-Quora-Dataset-Release-Question-Pairs.
  • [Kiros et al.2015] Ryan Kiros, Yukun Zhu, Ruslan R Salakhutdinov, Richard Zemel, Raquel Urtasun, Antonio Torralba, and Sanja Fidler. 2015. Skip-thought vectors. In Advances in Neural Information Processing Systems (NIPS).
  • [Lan and Xu2018] Wuwei Lan and Wei Xu. 2018. The importance of subword embeddings in sentence pair modeling. In Proceedings of the 2018 Conference of the North American Chapter of the Association for Computational Linguistics: Human Language Technologies (NAACL-HLT).
  • [Lan et al.2017] Wuwei Lan, Siyu Qiu, Hua He, and Wei Xu. 2017. A continuously growing dataset of sentential paraphrases. In Proceedings of The 2017 Conference on Empirical Methods on Natural Language Processing (EMNLP).
  • [Liu et al.2016] Pengfei Liu, Xipeng Qiu, and Xuanjing Huang. 2016. Modelling interaction of sentence pair with coupled-LSTMs. In Proceedings of the 2016 Conference on Empirical Methods in Natural Language Processing (EMNLP).
  • [Mikolov et al.2013] Tomas Mikolov, Ilya Sutskever, Kai Chen, Greg S Corrado, and Jeff Dean. 2013. Distributed representations of words and phrases and their compositionality. In Advances in neural information processing systems (NIPS).
  • [Nie and Bansal2017] Yixin Nie and Mohit Bansal. 2017. Shortcut-stacked sentence encoders for multi-domain inference. In Proceedings of the 2nd Workshop on Evaluating Vector Space Representations for NLP.
  • [Parikh et al.2016] Ankur Parikh, Oscar Täckström, Dipanjan Das, and Jakob Uszkoreit. 2016. A decomposable attention model for natural language inference. In Proceedings of the 2016 Conference on Empirical Methods in Natural Language Processing (EMNLP).
  • [Pennington et al.2014] Jeffrey Pennington, Richard Socher, and Christopher D. Manning. 2014. GloVe: Global vectors for word representation. In Proceedings of the 2014 Conference on Empirical Methods in Natural Language Processing (EMNLP).
  • [Rajpurkar et al.2016] Pranav Rajpurkar, Jian Zhang, Konstantin Lopyrev, and Percy Liang. 2016. SQuAD: 100,000+ questions for machine comprehension of text. In Proceedings of the 2016 Conference on Empirical Methods in Natural Language Processing (EMNLP).
  • [Rocktäschel et al.2016] Tim Rocktäschel, Edward Grefenstette, Karl Moritz Hermann, Tomáš Kočiskỳ, and Phil Blunsom. 2016. Reasoning about entailment with neural attention. In Proceedings of the International Conference on Learning Representations (ICLR).
  • [Shen et al.2017a] Gehui Shen, Yunlun Yang, and Zhi-Hong Deng. 2017a. Inter-weighted alignment network for sentence pair modeling. In Proceedings of the 2017 Conference on Empirical Methods in Natural Language Processing (EMNLP).
  • [Shen et al.2017b] Tao Shen, Tianyi Zhou, Guodong Long, Jing Jiang, Shirui Pan, and Chengqi Zhang. 2017b. Disan: Directional self-attention network for RNN/CNN-free language understanding. In

    Proceedings of the Association for the Advancement of Artificial Intelligence (AAAI)

    .
  • [Socher et al.2011] Richard Socher, Cliff C Lin, Chris Manning, and Andrew Y Ng. 2011. Parsing natural scenes and natural language with recursive neural networks. In Proceedings of the 28th International Conference on Machine Learning (ICML).
  • [Tomar et al.2017] Gaurav Singh Tomar, Thyago Duque, Oscar Täckström, Jakob Uszkoreit, and Dipanjan Das. 2017. Neural paraphrase identification of questions with noisy pretraining. In Proceedings of the First Workshop on Subword and Character Level Models in NLP.
  • [Wang and Jiang2017] Shuohang Wang and Jing Jiang. 2017. A compare-aggregate model for matching text sequences. In Proceedings of the International Conference on Learning Representations (ICLR).
  • [Wang et al.2007] Mengqiu Wang, Noah A Smith, and Teruko Mitamura. 2007. What is the Jeopardy model? A quasi-synchronous grammar for qa. In Conference on Empirical Methods in Natural Language Processing and Computational Natural Language Learning (EMNLP-CoNLL).
  • [Wang et al.2017] Zhiguo Wang, Wael Hamza, and Radu Florian. 2017. Bilateral multi-perspective matching for natural language sentences. In Proceedings of the Twenty-Sixth International Joint Conference on Artificial Intelligence (IJCAI).
  • [Wieting and Gimpel2017] John Wieting and Kevin Gimpel. 2017. Revisiting recurrent networks for paraphrastic sentence embeddings. In Proceedings of the 55th Annual Meeting of the Association for Computational Linguistics (ACL).
  • [Wieting et al.2016] John Wieting, Mohit Bansal, Kevin Gimpel, and Karen Livescu. 2016. Towards universal paraphrastic sentence embeddings. In Proceedings of the 4th International Conference on Learning Representations (ICLR).
  • [Williams et al.2017] Adina Williams, Nikita Nangia, and Samuel R Bowman. 2017. A broad-coverage challenge corpus for sentence understanding through inference. arXiv preprint arXiv:1704.05426.
  • [Xu et al.2014] Wei Xu, Alan Ritter, Chris Callison-Burch, William B. Dolan, and Yangfeng Ji. 2014. Extracting lexically divergent paraphrases from Twitter. Transactions of the Association for Computational Linguistics (TACL).
  • [Xu et al.2015] Wei Xu, Chris Callison-Burch, and William B. Dolan. 2015. SemEval-2015 Task 1: Paraphrase and semantic similarity in Twitter (PIT). In Proceedings of the 9th International Workshop on Semantic Evaluation (SemEval).
  • [Yang et al.2015] Yi Yang, Wen-tau Yih, and Christopher Meek. 2015. WikiQA: A challenge dataset for open-domain question answering. In Proceedings of the 2015 Conference on Empirical Methods in Natural Language Processing (EMNLP).
  • [Yin et al.2016] Wenpeng Yin, Hinrich Schütze, Bing Xiang, and Bowen Zhou. 2016. ABCNN: Attention-based convolutional neural network for modeling sentence pairs. Transactions of the Association for Computational Linguistics (TACL).

Appendix A Pretrained Word Embeddings

We used the 200-dimensional GloVe word vectors [Pennington et al.2014], trained on 27 billion words from Twitter (vocabulary size of 1.2 milion words) for Twitter URL [Lan et al.2017] and PIT-2015 [Xu et al.2015]

datasets, and the 300-dimensional GloVe vectors, trained on 840 billion words (vocabulary size of 2.2 milion words) from Common Crawl for all other datasets. For out-of-vocabulary words, we initialized the word vectors using normal distribution with mean

and deviation .

Appendix B Hyper-parameter Settings

We followed original papers or code implementations to set hyper-parameters for these models. In Infersent model [Conneau et al.2017], the hidden dimension size for Bi-LSTM is 2048, and the fully connected layers have 512 hidden units. In SSE model [Nie and Bansal2017], the hidden size for three Bi-LSTMs is 512, 2014 and 2048, respectively. The fully connected layers have 1600 units. PWIM [He and Lin2016] and ESIM [Chen et al.2017] both use Bi-LSTM for context encoding, having 200 hidden units and 300 hidden units respectively. The DecAtt model [Parikh et al.2016] uses three kinds of feed forward networks, all of which have 300 hidden units. Other parameters like learning rate, batch size, dropout rate, and all of them use the same settings as in original papers.

Appendix C Fine-tuning the Models

It is not practical to fine tune every hyper-parameter in every model and every dataset, since we want to show how these models can generalize well on other datasets, we need try to avoid fine-tuning these parameters on some specific datasets, otherwise we can easily get over-fitted models. Therefore, we keep the hyper-parameters unchanged across different datasets, to demonstrate the generalization capability of each model. The default number of epochs for training these models is set to 20, if some models could converge earlier (no more performance gain on development set), we would stop running them before they approached epoch 20. The 20 epochs can guarantee every model get converged on every dataset.