Semi-supervised Sentiment Analysis for Under-resourced Languages with a Sentiment Lexicon

Abstract

This paper presents the results of using semi-supervised sentiment analysis on an under-resourced language such as Norwegian. To perform these experiments, two external resources have been used: an available training corpus containing Norwegian reviews from major newspaper sources (NoRec) [23], and a newly created general sentiment lexicon for Norwegian, as presented in [12]. The results of our experiments show that the performance improves significantly when the sentiment lexicon is used. Besides, the best results are obtained using Support Vector Machines (SVM) as the machine learning algorithm used for training with an AUC score of around 92%. An alternative statistical measure was used for evaluation, Area Under ROC Curve (AUC), in order to deal with the highly imbalanced nature of the dataset.

CCS CONCEPTS

Information systems Sentiment analysis; Content analysis and feature selection; Language models;

INTRODUCTION

Sentiment analysis, or the automatic interpretation of the positive or negative orientation of a text, is now a widely used technique for several intelligent applications.

There are two main methods to do sentiment analysis. A widely used method is to use a big training corpus to train a supervised learning algorithm. The main challenge of this approach is cross-domain sentiment analysis. As soon as the trained model is used on diferent corpus the performance of the analysis drops abruptly. The second method makes use of a sentiment lexicon in order to perform sentiment analysis on any type of text. Very frequently a rule-based sentiment analysis algorithm is used in this approach, which simply averages the number and/or weights of the polarity words in the text.

A common challenge of both approaches is the lack of suficiently big and representative training corpora and sentiment lexicons. Despite the fact that the number of resources for the English language is enormous, the reality is that resources for other languages are still quite scarce. Training corpora Copyright ' 2019 for this paper by its authors. Use permitted under Creative Commons License Attribution 4.0 International (CC BY 4.0). require annotation and they are usually domain dependent. Besides, general sentiment lexicons are very expensive to build and in most languages they are not easily available.

The aim of this paper is two-fold. Firstly, we want to present the results of using semi-supervised machine learning on an available training corpus. Secondly, we seek to determine the impact of using a general sentiment lexicon for semi-supervised learning. We will perform these experiments on a low-resource Scandinavian language as Norwegian.

The contents of this paper are as follows. Related work is presented in Section 2. Then, the method presented in this paper is presented in Section 3 Our experiments and results are described in Section 4. This paper concludes with a brief discussion in Section 5. 2

RELATED

WORK

A number of machine learning and lexicon-based approaches for sentiment analysis have been proposed in recent research. With respect to the first technique, most approaches use classification algorithms to determine the polarity of a text, such as Support Vector Machines (SVM), Bayesian Networks, and decision trees, among others. For example, a supervised approach was presented by Habernal et al. [ 8 ] in which they explored mutual information, information gain, chi-square, odds ratio and relevancy score. SVM has been employed to attain 73.85 % f-measure. A manually tagged Facebook dataset was employed for evaluation which may be a source of bias. Sentiment polarity categorization using information theoretic approaches was explored by Lin et al. [ 11 ]. Approaches such as information gain, chi-square were applied in completely supervised experimental settings. The sentiment scores were computed by determining the correlation of a term with positive and negative labels, respectively. Term frequency was incorporated to intensify the feature weight. An accuracy ranging from 80.65 to 82.80 % was achieved on diferent product review datasets. However this approach is highly domain specific, needs labeled data for training and does not handle singularities. In turn, Singh and Husain [ 20 ] evaluated three supervised machine learning algorithms namely SVM, Naive Bayes and Multi-Layer Perceptron (MLP). The best performance results of 81.15 % accuracy were achieved for SVM on a movie review dataset. Each of these supervised algorithms has its pros and cons such that nominal attributes and missing values must be processed for SVM. Besides, Naive Bayes assumes attribute independence that might not always be the case, whereas MLP needs more training data and execution time.

Regarding lexicon-based techniques, SentiWordNet is one of the most generally used sentiment lexicons in the literature [ 14, 18 ]. This lexicon is based on WordNet and it contains multiple senses of a word. Besides, it provides a positive, objective, and negative value for each sense. In a semi-supervised approach, a sentiment sense inventory was built by Ortega et al. [ 16 ] based on SentiWordNet (SWN) scores. Rule-based labeling was employed to label the SWN scores into five categories. Adjectives, adverbs and verbs were utilized to achieve 50.17 % f1-score on tweets dataset. Low performance level and ignorance of nouns as semantic words are two of the major problems in this research. Ohana and Tierney [ 15 ] investigated sentiment orientation using SentiWordNet with SVM using adjectives, adverbs and verbs as candidate features in a semi-supervised manner. The feature weight was computed by considering the term position relative to the total number of terms in the document. They achieved 69.35 % accuracy on a movie review dataset. A constant value was manually adjusted to optimize the feature weight, and nouns were not included in the list of candidate features to be used as semantic words. Another semi-supervised approach was presented by Bhaskar et al. [ 3 ] in which they identified the emotions using WordNetAfect and SWN followed by SVM classification using term frequency in SVM vectors. SWN, SenticNet and a list of positive/negative words were incorporated with SVM by Chikersal et al. [ 4 ]. SentislangNet was constructed by Pandarachalil et al. [ 17 ] using SWN and SenticNet with a slangs dictionary. Ghosh and Kar [ 7 ] utilized adjectives adjacent to nouns as sentiment features based on SWN.

There are a number of lexical resources for English sentiment analysis, such as WordNet-Afect [ 22 ], SentiSense [ 6 ], Opinion Lexicon [ 10 ], Subjectivity Lexicon Rilof and Wiebe [ 19 ] and MPQA Opinion Corpus [ 24 ], etc. However, for underresourced languages like Norwegian, it is challenging to find training corpora or sentiment lexicons. Recently, Velldal et al. [ 23 ] have released a Norwegian Review corpus which can be used for evaluating sentiment analysis algorithms. This corpus will be used in our experiments. 3

METHOD

Similar to the general frameworks of sentiment analysis, the input to our pipeline are datasets from specific data sources and the output a unique polarity score for the input document. The overview of the proposed approach is presented in Figure 1. After receiving the data from data repositories, the framework transmits the data to a data pre-processing module. After these data are part-of-speech tagged, the tagged output is conveyed into the feature extraction module, that enriches the text with sentiment information from a sentiment lexicon. Finally, the classifier assigns the input test set with sentiment polarities after the model learning process. 3.1

Data pre-processing

In order to convert the unstructured data into machine readable format, an extensive pre-processing procedures is required. Specifically, we apply the following strategies: ∙ Stemming and lemmatization are general means to avoid diferent forms of a word to appear in the same document, especially when dictionary lookup needs to be performed. Stemming usually obtains the stem of the word by removing derivational axfies. In contrast, lemmatization reduces the word to its lemma by considering the use of a vocabulary and morphological analysis of the word. Even though we are aware that lemmatization can be more efective in the early stages of data pre-processing, lemmatizers are hard to find for the Norwegian language. For this reason, NLTK was used as a stemming tool1 to handle diferent word variants in Norwegian. ∙ Stop words are usually semantically empty, and thus they should be removed from the original documents. For Norwegian NLTK was used to perform this filtering2. ∙ Negations are also crucial in handling polarity shift problem in sentiment analysis. If negation appears in a sentence, we should consider if the sentiment score needs to be reversed or not. Thus, in this paper, before the removal of the stop words, we manually keep the following negations in Norwegian: ikke,ikkje, ei, nei, aldri, neppe, ingen, inga, intet, inkje, that respectively mean not, no, never, hardly, none, any. 3.2

Part of speech tagging

Part-of-Speech (POS) tagging is the process of assigning a morphosyntactic category to each word appearing in a given text. Identifying POS tags is also a key procedure to sentiment classification tasks as it can help to distinguish diferent sentiment polarities with diferent sentiment scores.

Most POS tagging tools are designed for English language. In other languages, such as Norwegian, these tools are scarce. Fortunately, the work in [ 12 ] ofers us a POS tagger for Norwegian bøkmal˚ , which will assign each word in the corpus with a tag. The tag will then be projected to sentiment lexicon tag according to the mapping defined in Table 1. Note that we do not incorporate adverbs in our experiments because there are no adverbs in the sentiment lexicon. Three kinds of features are extracted from the datasets by using a feature extraction module, namely TF-IDF, sentiment vector and statistical features.

Term Frequency Inverse Document Frequency (TF-IDF) is a popular and eficient scheme to determine how relevant a word is to a particular document. Intuitively, words commonly appear in a single or a small set of documents are more relevant/representative than words appears in most 1http://snowball.tartarus.org/algorithms/norwegian/stemmer.html 2https://github.com/xiamx/node-nltk-stopwords documents but with high term frequency. Such representative words are usually assigned with high TF-IDF score. On the contrary, commonly occurring words are always assigned a low TF-IDF score. Thus, the first feature is an input vector with the same length as the vocabulary size. Each element of the vector is set to a specific TF-IDF score if the word appears in the input document, and otherwise 0.

The second feature is a vector with the same length as the first feature (SV). In contrast, each element of the vector is assigned a particular sentiment score from the sentiment lexicon according to the word’s part-of-speech in the input document.

Some statistical features are also important for sentiment classification (SS). Specifically, in this paper, we make statistics on: 1) The minimum/maximum sentiment score of the input document. 2) The number of negative/positive words of the input document. 3) The sum of negative/positive score in the input document. 4) If the sum of negative score is higher than the positive score. 3.4

Algorithms

In this paper we have evaluated the results of four diferent machine learning algorithms that are generally used in text classification: Gaussian Naive Bayes (NB), Logistic Regression (LR), Support Vector Machine (SVM) and Neural Networks (NN). Implementations in the freely available package scikit-learn were used for these experiments.3

Machine learning algorithms have been widely used for sentiment analysis and text classification. Especially, SVM, that tries to find the maximum margin to separate classes, has been considered more appropriate than generative models for sentiment classification because it can diferentiate mixed sentiment better [ 5 ]. However, in [ 9 ], it is suggested that a Naive Bayes classifier might be more appropriate for small training data since SVM needs a large set of training data in order to achieve a high classification accuracy. Besides, researchers in [ 2 ] adopted NB for Norwegian political news sentiment classification and achieved comparatively good results.

The reason that we choose LR as one of our baselines lies in that similarly to NB and SVM, LR is a lightweight algorithm with relatively high computational speed. Even for some tasks, LR probably performs better than other more complicated algorithms.

Neural Networks, especially deep learning, are gaining lots of attention lately due to its superiority in terms of accuracy when trained on huge amount of data. Recent studies have already employed NN to solve large-scale unsupervised or semi-supervised sentiment classification, in which each layer of a deep neural network architecture represents features at a diferent level [ 13, 21 ]. However, it has not yet been utilized 3https://scikit-learn.org/stable/index.html for sentiment identifier in the Norwegian language. Thus, in this paper, we make an initial attempt and implement a fourlayer Multilayer Perceptron (MLP) with 100 units in each layer to perform the given tasks. The detailed experiments are described in Section 4. 4

EXPERIMENTS

In this section, we conduct our experiments on a real-world dataset. First, we introduce the datasets, sentiment lexicon, data preparation and evaluation metrics. Then we compare the performance of sentiment classification with diferent algorithms. After that, the efectiveness of various features proposed in this paper will be tested. 4.1

Data sets

Two resources were only used in these experiments: a training corpus and a sentiment lexicon.

4.1.1 Training corpus. The Norwegian Review Corpus (NoReC) is the training corpus used for these experiments.4 This corpus was created for the purpose of training and evaluating models for document-level sentiment analysis. The dataset contains more than 35,000 full-text reviews (approx. 15 million tokens) from Norwegian news sources and covering a range of domains, including literature, movies, video games, restaurants, music and theater, in addition to product reviews across a range of categories. In this dataset, each review is labeled with a manually assigned score of 1-6, as provided by the rating of the original author and following the Norwegian newspaper review tradition [ 23 ].

4.1.2 Sentiment lexicon. The lexicon used in this approach is a newly created general sentiment lexicon for the Norwegian language. In brief, the weights from SentiWordNet [ 1 ] were automatically transferred into the Danish WordNet, and the resulting resource was translated into Norwegian. The approach used to build this resource is explained in [ 12 ]. This lexicon contains 33,224 synsets and 35,035 wordsenses with information of their positive, negative or neutral polarity. Similarly to SentiWordnet, only 20% of the senses show positive or negative polarity. The distribution of synsets per morphological category in the lexicon is shown in Figure 2. 4.2

Data preparation

In order to perform the semi-supervised learning experiments, the review corpus was randomly split in a training, that amounts to 80% of the total review texts, and a test corpus, to 20%. To evaluate the efect of the type of text in the results, we performed experiments with two diferent versions of the review corpus. In the first experiments we used the full dataset, where reviews with 1, 2 and 3 review points were considered negative, and reviews with 4,5 and 6 were positive. In the second experiments, a simplified version of the review corpus was used, where only reviews with 1 and 2 review points were considered negative, and reviews with 5 and 6 positive. Reviews with 3 and 4 points were excluded 4https://github.com/ltgoslo/norec from the dataset in order to avoid the natural ambiguous language inherent to borderline cases in which the reviewer does not clearly express whether she has a fully positive or negative opinion on the product or service.

Some general statistics on these two kinds of datasets can be observed in Table 2. From the disproportionate ratio, we can see the sentiment polarity distribution is remarkably imbalanced across these two datasets, which will render the standard accuracy no longer reliable. There exists many ways to alleviate such phenomena, such as up-sampling, downsampling, change training strategy and so on. In this paper, we adopt down-sampling of our datasets with randomly removing observations from the majority class and keeping the same number of observations with the minority class. Meanwhile, instead of using the accuracy metric, we adopt Area Under ROC Curve (AUC), a well-known classification metric, to evaluate the performance. limited amount of training data. We think Neural Networks are more suitable for large-scale datasets and more complex problems.

Besides, it is interesting to observe that the performances on the simplified review corpus are better than the ones on the full version. As it was mentioned before, this might be the case because in the complete dataset, reviews rated by customers with 3 and 4 review points show ambiguous opinions which actually cannot be diferentiated so easily as positive or negative sentiment polarity, and thus bring extra noise to the model training process. In this section, we experiment on the efectiveness of classification performance with diferent feature combinations. The results are shown in Tables 4 and 5. We can see from both tables, the performances of SVM are superior than the other models on nearly all kinds of feature combinations on all datasets, which further verify the efectiveness of SVM model on sentiment classification.

In terms of features, similar patterns can be found on both datasets. Firstly, TF-IDF is the most important feature in our experiments because our model with TF-IDF solely achieves the best overall performance in AUC score than the model with SV or SS. Furthermore, the results deteriorate dramatically if only SV+SS are considered. On the other hand, statistical features (SS) have the lowest impact on sentiment classification for the model with TF-IDF+SV+SS improves the performance from the model with TF-IDF+SS a little but not much. Lastly, the model incorporating three input features outperforms the model with other feature combinations in AUC score suggests that all three kinds of features are still helpful in our tasks from diferent aspects. TF-IDF filters words with their representativeness according to TF-IDF scores in the first place. Apart from that, SV contributes to the sentiment distribution with part-of-speech appearing in sentiment lexicon. Finally, SS brings to the model useful patterns in the perspective of statistics. Therefore, our approach with TF-IDF+SV+SS presents the best AUC score in most cases in our experiments. 5

DISCUSSION AND FUTURE WORK

In this paper we investigate semi-supervised sentiment analysis using a sentiment lexicon for an under-resourced language as Norwegian. To our knowledge, this is the first paper that explores this challenge on Norwegian. The results of our experiments show that SVM perform the best. As expected, the use of features obtained from the general sentiment lexicon improves the results significantly. Interestingly, Neural Networks do not obtain competitive results. Our impression is that this might be the result of using a comparatively small dataset. We propose to use an alternative statistical measure to evaluate the performance of the machine learning algorithms, AUC, as the training corpus is highly imbalanced. 6

ACKNOWLEDGMENTS

This work was supported by the Research Council of Norway (grant number 245469).

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