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Semantic Textual Similarity

Yezheng Li     Danni Ma     Anant Maheshwari     Simeng Sun

Abstract

Semantic Textual Similarity (STS) measures the meaning similarity of sentences. Applications of this task include machine translation, summarization, text generation, question answering, short answer grading, semantic search, dialogue and conversational systems. We developed Support Vector Regression model with various features including the similarity scores calculated using alignment-based methods and semantic composition based methods. We have also trained sentence semantic representations with BiLSTM and Convolutional Neural Networks (CNN). The correlations between our system output the human ratings were above 0.8 in the test dataset.

Introduction

The goal of this task is to measure semantic textual similarity between a given pair of sentences (what they mean rather than whether they look similar syntactically). While making such an assessment is trivial for humans, constructing algorithms and computational models that mimic human level performance represents a difficult and deep natural language understanding (NLU) problem.

Example 1:

English: Birdie is washing itself in the water basin.

English Paraphrase: The bird is bathing in the sink.

Similarity Score: 5 ( The two sentences are completely equivalent, as they mean the same thing.)

Example 2:

English: The young lady enjoys listening to the guitar.

English Paraphrase: The woman is playing the violin.

Similarity Score: 1 ( The two sentences are not equivalent, but are on the same topic. )

Semantic Textual Similarity (STS) measures the degree of equivalence in the underlying semantics of paired snippets of text. STS differs from both textual entailment and paraphrase detection in that it captures gradations of meaning overlap rather than making binary classifications of particular relationships. While semantic relatedness expresses a graded semantic relationship as well, it is non-specific about the nature of the relationship with contradictory material still being a candidate for a high score (e.g., “night” and “day” are highly related but not particularly similar). The task involves producing real-valued similarity scores for sentence pairs. Performance is measured by the Pearson correlation of machine scores with human judgments.

STS is an annual shared task in SemEval since 2012. The STS shared tast data sets have been used extensively for research on sentence level similarity and semantic representations. We have access to STS benchmark which is a new shared training and evaluation set carefully selected from the corpus of English STS shared task data (2012-2017). Over the past five years, numerous participating teams, diverse approaches, and ongoing improvements to state-of-the-art methods have constantly raised the standard of this task.

Literature Review

The following paper summarizes the task for semantic textual similarity with a summary of each team's model which performed well in the competition. We try to review a few of those here. Cer, Daniel, et al. "SemEval-2017 Task 1: Semantic Textual Similarity-Multilingual and Cross-lingual Focused Evaluation." arXiv preprint arXiv:1708.00055 (2017)..

  1. ECNU

Junfeng Tian, Zhiheng Zhou, Man Lan, and Yuanbin Wu. 2017. ECNU at SemEval-2017 Task 1: Leverage kernel- based traditional nlp features and neural networks to build a universal model for multilingual and cross-lingual seman- tic textual similarity. In Proceedings of SemEval-2017.

This model is an average ensemble of three feature engineered models and four deep learning models. Three feature engineered models use Random Forest, Gradient Boosting and XGBoost regression methods which include approaches like edit distance and tree kernels. In neural network models, each network feeds the network feeds the element-wise multiplication, subtraction and concatenation of paired sentence embeddings.

  1. BIT

Wu, Hao, et al. "BIT at SemEval-2017 Task 1: Using semantic information space to evaluate semantic textual similarity." Proceedings of the 11th International Workshop on Semantic Evaluation (SemEval-2017). 2017.

This paper introduces three methods to measure semantic textual similarity(STS), all of which rely on jaccard coefficient of information content of the given sentence pairs. This problem can then be simplified to computing non-overlapping information content(IC) of two sentences. By adding words one by one from each layer of WordNet hierarchy taxonomy and computing information content gain iteratively, their original algorithm is improved since no searching for all subsume concepts is needed. Besides computing the similarity score using single IC feature in an unsupervised way, they also tried to combine sentence alignment and word embedding respectively as extra feature to train supervised models to improve their performance. According to their result, IC combined with word embedding achieves the best result.

  1. MITRE

John Henderson, Elizabeth Merkhofer, Laura Strickhart, and Guido Zarrella. 2017. MITRE at SemEval-2017 Task 1: Simple semantic similarity. In Proceedings of SemEval2017.

This paper took fourth place overall. The authors created an ensemble of following five systems which each independently predicted a similarity score.

  • TakeLab: They used LIBSVM to train a support vector regression model with an RBF kernel. Input features were comprised of TakeLab-computed n-gram overlap and word similarity metrics.
  • Recurrent Convolutional Neural Network: They extended RNN with four new feature categories. The first was a binary variable that indicates whether both words in the pair were determined to have the same dependency type in their respective parses. They also added three convolutional recurrent neural networks (CRNNs), each of which receive as input a sequence of word embeddings. For each aligned word pair, the first CRNN operates on the embeddings of the aligned words, the second CRNN operates on the squared difference of the embeddings of the aligned words, and the final CRNN operates on the embeddings of the parent words selected by the dependency parse.
  • Paris: String Similarity: Paris computes 22 string similarity metrics for a pair of strings, and expanded the number of features to 352. LIBLINEAR used these features to build a L1-regularized logistic regression model.
  • Simple Alignment Measures: They built an alignment system on top of word embeddings. L2-regularized logistic regression was used to combine 16 simple features calculated as side-effects of alignments.
  • Enhanced BiLSTM Inference Model (EBIM): They recreated a model which encodes each sentence with a bidirectional LSTM over word2vec embeddings, used a parameter-less attention mechanism to produce a soft alignment matrix for the two sentences, and then did inference over each timestep and its alignment using another LSTM. Two fully-connected layers completed the prediction.

The semantic similarity estimates of the predictors described above contributed to the final prediction with a weighting determined by L2-regularized logistic regression.

  1. FCICU

Basma Hassan, Samir AbdelRahman, Reem Bahgat, and Ibrahim Farag. 2017. FCICU at SemEval-2017 Task 1: Sense-based language independent semantic textual similarity approach. In Proceedings of SemEval-2017.

This paper computes a sense-base alignment using BabelNet and took fifth place overall. BabelNet synsets are multilingual allowing non-English and cross-lingual pairs to be processed similarly to English pairs. Alignment similarity scores are used with two runs: one that combines the scores within a string kernel and another that relied on the proportions of the aligned content words on the two sentences. Both runs average the Babelnet based scores with soft-cardinality. Specifically, the authors set the similarity score by taking the arithmetic mean of: 1) simproposed: a proposed sentence-pair semantic similarity score, and 2) simSC: the surfaced-based similarity function. They pre-processed multilingual input sentences with four different tasks: tokenization, stopwords removal, lemmatization and sense tagging.

  1. Compi_LIG

Ferrero, Jérémy, et al. "CompiLIG at SemEval-2017 Task 1: Cross-language plagiarism detection methods for semantic textual similarity." arXiv preprint arXiv:1704.01346 (2017).

  • Their system combined syntax-based, dictionary-based, context-based and MT-based methods in both supervised and unsupervised way. For syntactical method, they compute cosine similarity of n-gram representation of two sentences; for dictionary-based method, two sets of words for another language can be obtained from Google Translate, then the summation of weighted Jaccard distance of such set is used to compute the final score; for context-based method, they use weighted distributed representation of words as sentence embedding and compute cosine similarity, where the weights are computed in dictionary-based method; for MT-based approach, they use monolingual aligner to get aligned utterances and measure a variation of jaccard distance based on inverse document frequency of aligned utterances.
  1. LIM_LIG

Ferrero, Jérémy, and Didier Schwab. "LIM-LIG at SemEval-2017 Task1: Enhancing the Semantic Similarity for Arabic Sentences with Vectors Weighting." International Workshop on Semantic Evaluations (SemEval-2017). 2017.

  • CBOW model is the basic idea for word embeddings, with some modification. Besides idf weights, this manuscript includes POS weights.
  • From word embeddings to sentence embeddings, they use sum of vectors.
  1. DT_Team

Maharjan, Nabin, et al. "DT_Team at SemEval-2017 Task 1: Semantic Similarity Using Alignments, Sentence-Level Embeddings and Gaussian Mixture Model Output." Proceedings of the 11th International Workshop on Semantic Evaluation (SemEval-2017). 2017.

  • unsupervised and supervised learning;
  • POS-tagging, name-entity recognition as well as normalization, tokenization, lemmatization are preprocess procedure for word embeddings;
  • From word embeddings to sentence embeddings, this manuscript describes (interesting to me) word alignment; then the similarity score was computed as the sum of the scores for all aligned word-pairs divided by the total length of the given sentence pair.
  1. sent2vec

Pagliardini, Matteo, Prakhar Gupta, and Martin Jaggi. "Unsupervised learning of sentence embeddings using compositional n-gram features." arXiv preprint arXiv:1703.02507 (2017).

  • This is unsupervised learning (not using true similarities in gs file) with similarities of each pair just from cosine similarity of sentence embeddings. The sentence embeddings is sent2vec.

Experimental Design

Data

We obtained the data by merging data from year 2012 to 2017 SemEval Shared Task. Out of a total of approximately 28000 sentence pairs, we were left with about 15115 sentence pairs (cleaning involved removing sentence pairs without a tab delimiter and pairs with a blank gold score). We split the data into three parts as below.

Training data: 13365 pairs Validation data: 1500 pairs Test data: 250 pairs (Same as used by the other teams to test their model in the 2017 task)

Data Pre-processing

We used tokenization and lemmatization on the data as a pre-processing step before we turn the data into our models. We chose to do this step as lemmatization does not take away any semantic information from sentences and hence was an essential step for our application.

Evaluation Metric

The official score is based on weighted Pearson correlation between predicted similarity and human annotated similarity. The higher the score, the better the the similarity prediction result from the algorithm.

Simple baseline

For the simple baseline, we used an unsupervised approach by creating sentence vectors with each dimension representing whether an individual word appears in a sentence. The final score is calculated using cosine similairty between the sentence vectors.

We achieved the following results using the simple baseline:

Model Validation Data Test Data
Simple baseline 0.428 0.633

Experimental Results

Published baseline

We re-implemented DT_Team's work[2], as this is the state-of-the-art for monolingual STS task. DT_Team used linear Support Vector Regression with a set of 7 features. Among them, two were only compatible with Windows, and one feature did not improve the performance at all, so we re-implemented four of them. Details are below.

  1. Unigram Overlap: Count the unigram overlap between sentence A and sentence B, with synonym check. Then Calculate the ration of overlap count over the total length of two sentences.
  2. Word Alignment: Look for each word of sentence A in Wordnet, we can get a synonym set (synset). We align this word with a corresponding one in sentence B by using path similarity in Wordnet trees. The labeled alignments are used as one useful feature.
  3. Absolute Difference: Let Cta and Cta be the counts of tokens of type t ∈ {all tokens, adjectives, adverbs, nouns, and verbs} in sentence A and B respectively. We calculate the absolute difference as |Cta−Ctb|/(Cta+Ctb).
  4. Min to Max Ratio: Let Ct1 and Ct2 be the counts of type t ∈ {all, adjectives, adverbs, nouns, and verbs} for shorter Sentence 1 and longer Sentence 2 respectively. We calculate the minimum to maximum ratio Ct1/Ct2 as one feature.

The published performance of DT_Team model on the same test data is 0.8536. However, we could only achieve 0.6989 after re-implementation. One reason is that there are two features which might be useful, but we did not use due to operating system compatibility. It is also possible that DT_Team had some pre-processed steps which they did not illustrate very specifilly in the paper, and we did not fully implement them. However, we focus on our extensions in an attempt to reach close to the state of the art performance.

Model Validation Data Test Data
Simple baseline 0.428 0.633
Published baseline 0.6114 0.6989

Table: Pearson Correlations between system outputs and human ratings on different models

Extensions

  1. Resnik Similarity using Information Content from Brown Corpus

We used information content generated from the Brown corpus to compute the resnik similarity between paths in the wordnet trees for the given sentence pairs. This approach uses IC of the Least Common Subsumer (most specific ancestor node) to output a score which is used by the Support Vector Regression model.

We were able to improve upon our model by a slight amount using this extension:

Model Validation Data Test Data
baseline 0.6114 0.6989
baseline + IC 0.6226 0.7097

Table: Pearson Correlations between system outputs and human ratings on different models

  1. Convolution Neural Networks to generate sentence embeddings

alt text

The 2nd extension we implemented is a Convolutional neural network, which produces dense representation of sentences as by-product. We have two version of CNN model for this extension.

The first version is shown above. We stack the two sentence together by column and form a matrix of size N by 2D, D is the dimension of word embedding, N is the maximal length of sentence the model can take in. We use pretrained GoogleNews vector, D equals to 300. When the length of sentence is not long enough, we employ masks to cover the empty space. After convolution operation with kernel sizes range from 2 to 5, each with 16 output channels, a dense vector that encodes the semantic relationship of two sentences is generated. We then combine our baseline features and feed this vector in a linear regression model to predict similarity score.

The second version is shown below. Unlike the first version, we use CNN to encode each sentence separately and concatenate two sentence embedding and baseline features, then feed this new vector into a linear regression model. This model performs better than the last one, the best result we got is shown in the table below.

alt text

Model Validation Data Test Data
baseline 0.6114 0.6989
baseline + CNN 0.6615 0.6460

Table: Pearson Correlations between system outputs and human ratings on different models

  1. Use InferSent trained sentence embeddings

InferSent is a sentence embeddings method that provides semantic sentence representations. It is trained on natural language inference data and generalizes well to many different tasks.

We use InferSent to get the embeddings of all the sentences we have. Given a pair of sentences, if they are semantically similar, the cosine similary between two sentence embeddings are supposed to be high. We extracted the cosine similarity between sentence pairs, added it as a feature, and fed to our Support Vector Regression model.

With the help of InferSent trained sentence representations, the model outperforms baseline model on both validation data and test data:

Model Validation Data Test Data
baseline 0.6114 0.6989
baseline + InferSent 0.7220 0.8104

Table: Pearson Correlations between system outputs and human ratings on different models

Error Analysis

Altough Pearson correlation is widely used as evaluation criteria in the literature, it is not quite intepretable. We introduce two extra error analysis:

  1. "Ave 5": average over predicted similarities of pairs with gold standard 5.0. The closer "Ave 5" to 5.0, the better the model.
  2. "Ave 0": average over predicted similarities of pairs with gold standard 0.0. closer "Ave 5" to 0.0, the better the model.
Validation Set Test Set
Pearson Ave 5(128) Ave 0(131) Pearson Ave 5(10) Ave 0(19)
Simple Baseline 0.428 3.274 0.532 0.633 4.088 0.623
Published Baseline 0.611 3.994 0.668 0.698 4.347 0.861
CNN 0.661 4.030 0.569 0.646 3.767 0.561
LSTM 0.722 4.076 0.258 0.810 4.427 0.516
Gold Standard 1 5 0 1 5 0

Some observations:

  1. We can see the trend is that (generally) "Ave 5" and "Ave 0" are improved as Pearson correlation is improved; level of improvement may vary: take (Validation Set of CNN and LSTM) as an example, "Ave 0" is improved from 0.569 to 0.258 while "Ave 5" is only improved from 4.030 to 4.076.
  2. Since Test Set has only 250 pairs compaired to Validation Set (1500 pairs), we get only 10 pairs with gold standard 5 and only 19 pairs with gold standard 0 -- Test Set's result varies a lot -- take (Test Set of Published and CNN) as an example, Pearson correlation and "Ave 5" drop while "Ave 0" is improved. To conclude, "Ave 5" and "Ave 0" are less convincing since Test Set is not big.

Conclusions

The state-of-art performs for this particular task is a test score of 0.85 on the test set for 2017. With our baseline features and best performing extension, we were able to reach a score of 0.8104 on the test set. We tried many approaches involving binary bag-of-words, word2vec embeddings, fastext embeddings for en-es cross-lingual pairs and sent2vec embeddings. Binary bag-of-words, although being a very simple model tends to perform very well in general and is a high-performing baseline. We also tried lots of different features involving path-similarity and resnik similarity in wordnet trees for the given sentences. One would hope that CNN's would perform well by capturing semantic information in the generated embeddings but are still unable to beat the best score we obtained. However as shown in the error analysis, CNN does have a good potential on this application and would certainly outperform other models if more data is available.

References

[1] Cer et. al, SemEval-2017 Task 1: Semantic Textual Similarity Multilingual and Cross-lingual Focused Evaluation. In Proceedings of the 11th International Workshop on Semantic Evaluations (SemEval-2017)

[2] Maharjan et. al, DT Team at SemEval-2017 Task 1: Semantic Similarity Using Alignments, Sentence-Level Embeddings and Gaussian Mixture Model Output. In Proceedings of the 11th International Workshop on Semantic Evaluations (SemEval-2017)

[3] Banjade et. al, DTSim at SemEval-2016 Task 1: Semantic Similarity Model Including Multi-Level Alignment and Vector-Based Compositional Semantics. In Proceedings of SemEval-2016

[4] Conneau et. al, 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)

Acknowledgements

We took part in regular meetings with out mentor TA Nitish Gupta who helped us with his thoughts on our ideas and giving us possible directions for our extensions to improve our results.

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