Introduction
One of the crucial tasks for researchers to carry out scientific investigations is to detect existing ideas that are related to their research topics. Research ideas are usually documented in scientific publications. Normally, there is one main idea stated in the abstract, explicitly presenting the aim of the paper. There are also other sub-ideas distributed across the entire paper. As the growth rate of scientific publication has been rising dramatically, researchers are overwhelmed by the explosive information. It is almost impossible to digest the ideas contained in the documents emerged everyday. Therefore, computer assisted technologies such as document summarization are expected to play a role in condensing information and providing readers with more relevant short texts. Unlike document summarization from news circles, where the task is to identify centroid sentences BIBREF0 or to extract the first few sentences of the paragraphs BIBREF1 , summarization of scientific articles involves extra text processing stage BIBREF2 . After highest ranked texts are extracted, rhetorical status analysis will be conducted on the selected sentences. Rhetorical sentence classification, also known as argumentative zoning (AZ) BIBREF3 , is a process of assigning rhetorical status to the extracted sentences. The results of AZ provide readers with general discourse context from which the scientific ideas could be better linked, compared and analyzed. For example, given a specific task, which sentences should be shown to the reader is related to the features of the sentences. For the task of identifying a paper's unique contribution, sentences expressing research purpose should be retrieved with higher priority. For comparing ideas, statements of comparison with other works would be more useful. Teufel et. al. BIBREF2 introduced their rhetorical annotation scheme which takes into account of the aspects of argumentation, metadiscourse and relatedness to other works. Their scheme resulted seven categories of rhetorical status and the categories are assigned to full sentences. Examples of human annotated sentences with their rhetorical status are shown in Table. TABREF2 . The seven categories are aim, contrast, own, background, other, basis and textual.
Analyzing the rhetorical status of sentences manually requires huge amount of efforts, especially for structuring information from multiple documents. Fortunately, computer algorithms have been introduced to solve this problem. With the development of artificial intelligence, machine learning and computational linguistics, Natural Language Processing (NLP) has become a popular research area BIBREF4 , BIBREF5 . NLP covers the applications from document retrieval, text categorization BIBREF6 , document summarization BIBREF7 to sentiment analysis BIBREF8 , BIBREF9 . Those applications are targeting different types of text resources, such as articles from social media BIBREF10 and scientific publications BIBREF2 . There are several approaches to tackle these tasks. From machine learning prospective, text can be analysed via supervised BIBREF2 , semi-supervised BIBREF11 and unsupervised BIBREF12 algorithms.
Document summarization from social media and news circles has received much attention for the past decades. Those problems have been addressed from many angles, one of which is feature extraction and representation. At the early stage of document summarization, features are usually engineered manually. Although the hand-crafted features have shown the ability for document summarization and sentiment analysis BIBREF13 , BIBREF9 , there are not enough efficient features to capture the semantic relations between words, phrases and sentences. Moreover, building a sufficient pool of features manually is difficult, because it requires expert knowledge and it is time-consuming. Teufel et. al. BIBREF2 have built feature pool of sixteen types of features to classify sentences, such as the position of sentence, sentence length and tense. Widyantoro et. al. used content features, qualifying adjectives and meta-discourse features BIBREF14 to explore AZ task. It took efforts to engineer these features and it is also time consuming to optimize the combination of the entire features. With the advent of neural networks BIBREF15 , it is possible for computers to learn feature representations automatically. Recently, word embedding technique BIBREF16 has been widely used in the NLP community. There are plenty of cases where word embedding and sentence representations have been applied to short text classification BIBREF17 and paraphrase detection BIBREF18 . However, the effectiveness of this technique on AZ needs further study. The research question is, is it possible to extract word embeddings as features to classify sentences into the seven categories mentioned above using supervised machine learning approach?
Related Work
The tool of word2vec proposed by Mikolov et al. BIBREF16 has gained a lot attention recently. With word2vec tool, word embeddings can be learnt from big amount of text corpus and the semantic relationships between words can be measured by the cosine distances between the vectors. The idea behind word embeddings is to use distributed representation BIBREF19 to map each word into k-dimension vector. How these vectors are generated using word2vec tool? The common method to derive the vectors is using neural probabilistic language model BIBREF20 . The underlying word representations for each word are obtained while training the language model. Similar to the mechanism in language model, Mikolov et al. BIBREF16 introduced two architectures: Skip-gram model and continuous bag of words (CBOW) model. Each of the model has two different training strategies, such as hierarchical softmax and negative sampling. Both these two models have three layers: input, projection and output layer. The word vectors are obtained once the models are optimized. Usually, this optimizing process is done using stochastic gradient descent method. It doesn't need labels when training the models, which makes word2vec algorithm more valuable compared with traditional supervised machine learning methods that require a big amount of annotated data. Given enough text corpus, the word2vec can generate meaningful representations.
Word2vec has been applied to sentiment analysis BIBREF21 , BIBREF22 , BIBREF23 and text classification BIBREF24 . Sadeghian and Sharafat BIBREF25 explored averaging of the word vectors in a sentiment review statement. Their results indicated that word2vec models significantly outperform the vanilla bag-of-words model. Amongst the word2vec based models, softmax provides the best form of classification. Tang et al. BIBREF21 used the concatenation of vectors derived from different convolutional layers to analyze the sentiment statements. They also trained sentiment-specific word embeddings to improve the twitter sentiment classification results. This work is aiming at learning word embeddings for the task of AZ. The results were compared from three aspects: the impact of the training corpus, the effectiveness of specific word embeddings and different ways of constructing sentence representations based on the learned word vectors.
Le and Mikolov BIBREF26 introduced the concept of word vector representation in a formal way:
Given a sequence of training words INLINEFORM0 , the objective of the word2vec model is to maximize the average log probability:
INLINEFORM0 INLINEFORM1 INLINEFORM2 p INLINEFORM3 (1)
Using softmax technique, the prediction can be formalized as:
p INLINEFORM0 = INLINEFORM1 (2)
Each of INLINEFORM0 is un-normalized log probability for each output word INLINEFORM1 :
INLINEFORM0 (3)
Models
In this study, sentence embeddings were learned from large text corpus as features to classify sentences into seven categories in the task of AZ. Three models were explored to obtain the sentence vectors: averaging the vectors of the words in one sentence, paragraph vectors and specific word vectors.
The first model, averaging word vectors ( INLINEFORM0 ), is to average the vectors in word sequence INLINEFORM1 . The main process in this model is to learn the word embedding matrix INLINEFORM2 :
INLINEFORM0 INLINEFORM1 INLINEFORM2 INLINEFORM3 (4)
where INLINEFORM0 is the word embedding for word INLINEFORM1 , which is learned by the classical word2vec algorithm BIBREF16 .
The second model, INLINEFORM0 , is aiming at training paragraph vectors. It is also called distributed memory model of paragraph vectors (PV-DM) BIBREF26 , which is an extension of word2vec. In comparison with the word2vec framework, the only change in PV-DM is in the equation (3), where INLINEFORM1 is constructed from INLINEFORM2 and INLINEFORM3 , where matrix INLINEFORM4 is the word vector and INLINEFORM5 holds the paragraph vectors in such a way that every paragraph is mapped to a unique vector represented by a column in matrix INLINEFORM6 .
The third model is constructed for the purpose of improving classification results for a certain category. In this study specifically, the optimization task was focused on identifying the category INLINEFORM0 . In this study, INLINEFORM1 specific word embeddings were trained ( INLINEFORM2 ) inspired by Tang et al. BIBREF21 's model: Sentiment-Specific Word Embedding (unified model: INLINEFORM3 ). After obtaining the word vectors via INLINEFORM4 , the same scheme was used to average the vectors in one sentence as in the model INLINEFORM5 .
Classification and evaluation
The learned word embeddings are input into a classifier as features under a supervised machine learning framework. Similar to sentiment classification using word embeddings BIBREF21 , where they try to predict each tweet to be either positive or negative, in the task of AZ, the embeddings are used to classify each sentence into one of the seven categories.
To evaluate the classification performance, precision, recall and F-measure were computed.
Training Dataset
INLINEFORM0 collection. ACL Anthology Reference Corpus contains the canonical 10,921 computational linguistics papers, from which 622,144 sentences were generated after filtering out sentences with lower quality.
INLINEFORM0 collection contains 6,778 sentences, extracted from the titles and abstracts of publications provided by WEB OF SCIENCE .
Test Dataset
Argumentative Zoning Corpus ( INLINEFORM0 corpus) consists of 80 AZ INLINEFORM1 annotated conference articles in computational linguistics, originally drawn from the Cmplg arXiv. . After Concatenating sub-sentences, 7,347 labeled sentences were obtained.
Training strategy
To compare the three models effectiveness on the AZ task, the three models on a same ACL dataset (introduced int he dataset section) were trained. The word2vec were also trained using different parameters, such as different dimension of features. To evaluate the impact from different domains, the first model was trained on different corpus.
The characteristics of word embeddings based on different model and dataset are listed in Table. TABREF12 .
Parameters
Inspired by the work from Sadeghian and Sharafat BIBREF25 , the word to vector features were set up as follows: the Minimum word count is 40; The number of threads to run in parallel is 4 and the context window is 10.
Strategy of dealing with unbalanced data
In imbalanced data sets, some classes are significantly outnumbered by other classes BIBREF27 , which affects the classification results. In this experiment, the test dataset is an imbalanced data set. Table. TABREF16 shows the distribution of rhetorical categories from the INLINEFORM0 test dataset. The categories OWN and OTH are significantly outnumbering other categories.
To deal with the problem of classification on unbalanced data, synthetic Minority Over-sampling TEchnique (SMOTE) BIBREF28 were performed on the original dataset. 10-cross validation scheme was adopted and the results were averaged from 10 iterations.
Results of classification for per category
Table. TABREF19 and TABREF20 show the classification performance of different methods.
The results were examined from the following aspects:
When the feature dimension is set to 100 and the training corpus is ACL, the results generated by different models were compared (AVGWVEC,
PARAVEC and AVGWVEC+BSWE for BAS category only). Looking at the F-measure, AVGWVEC performs better than PARAVEC, but PARAVEC gave a better precision results on several categories, such as AIM, CTR, TXT and OWN. The results showed that PARAVEC model is not robust, for example, it performs badly for the category of BAS. For specific category classification, take the BAS category for example, the BSWE model outperforms others in terms of F-measure.
When the model is fixed to AVGWVEC and the training corpus is ACL, the feature size impact (300 and 100 dimensions) was investigated. From the F-measure, it can be seen that for some categories, 300-dimension features perform better than the 100-dimension ones, for example, CTR and BKG, but they are not as good as 100-dimension features for some categories, such as BAS.
When the model is set to AVGWVEC and the feature dimension is 100, the results computed from different training corpus were compared (ACL+AZ, MixedAbs and Brown corpus). ACL+AZ outperforms others and brown corpus is better than MixedAbs for most of the categories, but brown corpus is not as good as MixedAbs for the category of OWN.
Finally, the results were compared between word embeddings and the methods of cuewords, Teufel 2002 and baseline. To evaluate word embeddings on AZ, the model AVGWVEC trained on ACL+AZ was used for the comparison. It can be seen from the table. TABREF19 , the model of word embeddings is better than the method using cuewords matching. It also outperforms Teufel 2002 for most of the cases, except AIM, BAS and OWN. It won baseline for most of the categories, except OWN.
Discussion
The classification results showed that the type of word embeddings and the training corpus affect the AZ performance. As the simple model, INLINEFORM0 performs better than others, which indicate averaging the word vectors in a sentence can capture the semantic property of statements. By training specific argumentation word embeddings, the performance can be improved, which can be seen from the case of detecting BAS status using INLINEFORM1 model.
Feature dimension doesn't dominate the results. There is no significant difference between the resutls generated by 300-dimension of features and 100 dimensions.
Training corpus affects the results. ACL+AZ outperforming others indicates that the topics of the training corpus are important factors in argumentative zoning. Although Brown corpus has more vocabularies, it doesn't win ACL+AZ.
In general, the classification performance of word embeddings is competitive in terms of F-measure for most of the categories. But for classifying the categories AIM, BAS and OWN, the manually crafted features proposed by Teufel et al. BIBREF2 gave better results.
Conclusion
In this paper, different word embedding models on the task of argumentative zoning were compared . The results showed that word embeddings are effective on sentence classification from scientific papers. Word embeddings trained on a relevant corpus can capture the semantic features of statements and they are easier to be obtained than hand engineered features.
To improve the sentence classification for a specific category, integrating word specific embedding strategy helps. The size of the feature pool doesn't matter too much on the results, nor does the vocabulary size. In comparison, the domain of the training corpus affects the classification performance.

Question: What metric is considered?
Answer: Precision, recall and F-measure.

precision, recall, F-measure

precision, recall and F-measure

Question: What hand-crafted features are used?
Answer: position of sentence, sentence length, tense, qualifying adjectives, meta-discourse features

 sentences with their rhetorical status 

Unanswerable

Question: What word embeddings are used?
Answer: INLINEFORM1 specific word embeddings were trained ( INLINEFORM2 )

Sentiment-Specific Word Embedding, word2vec

word2vec, Sentiment-Specific Word Embedding

Question: Do they annotate their own dataset?
Answer: No

No

No

Question: How are the sentence embeddings generated?
Answer: sentence embeddings were learned from large text corpus as features to classify sentences into seven categories in the task of AZ. Three models were explored to obtain the sentence vectors: averaging the vectors of the words in one sentence, paragraph vectors and specific word vectors

Averaging the vectors of the words in a sentence, directly learning paragraph vectors using PV-DM, taking average of the SSWE of the words in a sentence.

 average the vectors in word sequence, training paragraph vectors, Sentiment-Specific Word Embedding

Question: What is argumentative zoning?
Answer:  Rhetorical sentence classification, also known as argumentative zoning (AZ) BIBREF3 , is a process of assigning rhetorical status to the extracted sentences

process of assigning rhetorical status to the extracted sentences

a process of assigning rhetorical status to the extracted sentences