Abstract: This paper presents a study of employing Ranking SVM and Convolutional Neural Network for two missions: legal information retrieval and question answering in the Competition on Legal Information Extraction/Entailment. For the first task, our proposed model used a triple of features (LSI, Manhattan, Jaccard), and is based on paragraph level instead of article level as in previous studies. In fact, each single-paragraph article corresponds to a particular paragraph in a huge multiple-paragraph article. For the legal question answering task, additional statistical features from information retrieval task integrated into Convolutional Neural Network contribute to higher accuracy.

Introduction: Legal text, along with other natural language text data, e.g. scientific literature, news articles or social media, has seen an exponential growth on the Internet and in specialized systems. Unlike other textual data, legal texts contain strict logical connections of law-specific words, phrases, issues, concepts and factors between sentences or various articles. Those are for helping people to make a correct argumentation and avoid ambiguity when using them in a particular case. Unfortunately, this also makes information retrieval and question answering on legal domain become more complicated than others. There are two primary approaches to information retrieval (IR) in the legal domain BIBREF0 : manual knowledge engineering (KE) and natural language processing (NLP). In the KE approach, an effort is put into translating the way legal experts remember and classify cases into data structures and algorithms, which will be used for information retrieval. Although this approach often yields a good result, it is hard to be applied in practice because of time and financial cost when building the knowledge base. In contrast, NLP-based IR systems are more practical as they are designed to quickly process terabytes of data by utilizing NLP techniques. However, several challenges are presented when designing such system. For example, factors and concepts in legal language are applied in a different way from common usage BIBREF1 . Hence, in order to effectively answer a legal question, it must compare the semantic connections between the question and sentences in relevant articles found in advance BIBREF2 . Given a legal question, retrieving relevant legal articles and deciding whether the content of a relevant article can be used to answer the question are two vital steps in building a legal question answering system. Kim et al. BIBREF2 exploited Ranking SVM with a set of features for legal IR and Convolutional Neural Network (CNN) BIBREF3 combining with linguistic features for question answering (QA) task. However, generating linguistic features is a non-trivial task in the legal domain. Carvalho et al. BIBREF1 utilized n-gram features to rank articles by using an extension of TF-IDF. For QA task, the authors adopted AdaBoost BIBREF4 with a set of similarity features between a query and an article pair BIBREF5 to classify a query-article pair into “YES" or “NO". However, overfitting in training may be a limitation of this method. Sushimita et al. BIBREF6 used the voting of Hiemstra, BM25 and PL2F for IR task. Meanwhile, Tran et al. BIBREF7 used Hidden Markov model (HMM) as a generative query model for legal IR task. Kano BIBREF8 addressed legal IR task by using a keyword-based method in which the score of each keyword was computed from a query and its relevant articles using inverse frequency. After calculating, relevant articles were retrieved based on three ranked scores. These methods, however, lack the analysis of feature contribution, which can reveal the relation between legal and NLP domain. This paper makes the following contributions: In the following sections, we first show our idea along with data analysis in the context of COLIEE. Next, we describe our method for legal IR and legal QA tasks. After building a legal QA system, we show experimental results along with discussion and analysis. We finish by drawing some important conclusions.

Question: What contribute to improve the accuracy on legal question answering task?
Answer: Adding more features to the traditional sets such as TF-IDF, BM25 and PL2F as well as using voting in a ranking system help to improve accuracy on a legal question answering task