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A New Semantic Lexicon and Similarity Measure in Bangla

The Mental Lexicon (ML) refers to the organization of lexical entries of a language in the human mind.A clear knowledge of the structure of ML will help us to understand how the human brain processes language. The knowledge of semantic association among the words in ML is essential to many applications. Although, there are works on the representation of lexical entries based on their semantic association in the form of a lexicon in English and other languages, such works of Bangla is in a nascent stage. In this paper, we have proposed a distinct lexical organization based on semantic association between Bangla words which can be accessed efficiently by different applications. We have developed a novel approach of measuring the semantic similarity between words and verified it against user study. Further, a GUI has been designed for easy and efficient access.


Introduction
The lexicon of a language is a collection of lexical entries consisting of information regarding words and expressions, comprising both form and meaning (Levelt,). Form refers to the orthography, phonology and morphology of the lexical item and Meaning refers to its syntactic and semantic information.
The term Mental Lexicon refers to the organization and interaction of lexical entries of a language in the human mind. Depending on the definition of word, an adult knows and uses around 40000 to 150000 words. Yet, it has been estimated that an adult can recognize a word in her native language in less than 200ms and can reject a non-word in less than 500ms (Aitchison, 2012;Muller, 2008;Seashore and Eckerson, 1940). Therefore, the storage and retrieval mechanisms of the brain have to be efficient enough to facilitate such super-fast access. Words in the mental lexicon are assumed to be associated at various levels of linguistic features such as, orthography, phonology, morphology and semantics. Although a vast amount of research is going on the mental lexicon, the precise natures of the relations are yet to be explored. The knowledge of semantic association among the words in mental lexicon is essential to many areas such as, developing pedagogical strategies, categorization, semantic web, natural language processing applications like, document clustering, word sense disambiguation, machine translation, information retrieval, text comprehension, and question-answering systems, where the perception of the target user group plays an important role.. However, as we cannot 'look into the mind' to know the exact structure of the mental lexicon, we try to simulate its behaviour with the help of external models.
The rich repertoire of literature on the structure, organization and representation of lexical entries includes simple organization schemes like Dictionary and Thesaurus to more complex ones like WordNet (Fellbaum, 2010) and ConceptNet (Liu and Singh, 2004) and also methods to measurethe degree of semantic similarity among the lexemes.
Bangla is an Indo-Aryan language having about 193 million native and about 230 million total speakers. Despite being so popular, very few attempts have (Roy and Muqtadir, 2008;Das and Bandyopadhyay, 2010) been made to build a semantically organized lexicon of substantial size in Bangla. Hence, we propose a distinct lexical organization.
The objective of this work is to design and develop a Bangla lexicon based on semantic similarity among Bangla words, which is suitable of automatic access mechanismsand can be used further in various applications like as mentioned above. The design is based on the Samsad Samarthasabdokosh (Mukhopadhyay, 2005). The lexicon is hierarchically organized and divided according to the categories or domains represented by different segments. The categories are further divided into sub-categories. The words are grouped into clusters along with their synonyms. Weighted edges between different types of words related to same or different concepts or categories exist, denoting the semantic distance between them. We have also developed a Graphical User Interface on top of the lexicon, which can be used for efficient and easy access. This is an on-going project with an aim of creating an organization containing 50,000 words.
The organization of the paper is as follows: section 2 contains the related works; we have also pointed out some of the differences of our proposed structure with WordNet in section 2; section 3 explains the construction of the lexicon and the GUI; section 4, describes the proposed approach of predicting semantic similarity between words; in section 5 we have discussed the user study; conclusions and future thoughts have been included in the last section.

Related work
A number of works have been done semantic relation based representations include simple organizational schemes like Dictionary and Thesaurus to more complex ones like WordNet (Fellbaum, 2010) and ConceptNet (Liu and Singh, 2004) and others (Ruppenhofer et al., 2010).Words in WordNet are organized around semantic groupings called synsets. Each synset consists of a list of synonymous word forms and semantic pointers that describe relationships among the synsets.However, WordNet suffers from several limitations (Boyd-Graber et al., 2006).ConceptNet is a semantic network containing different types of concepts and relationships among them. Here, concepts are represented by words or short phrases and relationships can be of many kinds such as, MotivatedByGoal, UsedFor, can cross.
According to the most recent reference to a Bangla WordNet (Roy and Muqtadir, 2008), the structure is based on Bangla to English bi-lingual dictionaries and in strict alignment (only the synonym equivalents are used) with the Princeton WordNet for English. It contains around 639 synsets and 1,455 words 1 . The assumptions that have been taken are: Bangla and English have significant amount of linguistic similarities and Bangla word senses can be clearly justified by a Bangla-English-Bangla dictionary.
Our proposed lexical representation is different from WordNet in many respects. Some of the important differences being: • No cross parts of speech links are there in the WordNet. That means no link between an entity and its attributes. are also under same node [weapon] but in different clusters. These kinds of relations can be helpful in word sense disambiguation applications. • Relational links are qualitative rather than quantitative in WordNet.. In our system we have given weight on different type of links keeping in mind the semantic closeness of the nodes they connect. Moreover, in our structure, there exists a path between each possible word pair.
Our proposed semantic similarity based lexical organization is not a substitution of WordNet; rather it tries to address some of the aspects which are still not incorporated in the WordNet framework. It is useful especially in case of a resource poor language like Bangla.

Work on measuring semantic similarity among words
There exist many approaches to measure semantic similarity between words; some of them are discussed here. Tversky's feature based similarity model (Tversky, 1977), is among the early works in this field. Some scholars (Rada et al., 1989;Kim and Kim, 1990;Lee et al., 1993) have proposed the conceptual distance approach that uses edge weights, between adjacent nodes in a graph as an estimator of semantic similarity. Resnick (Resnik, 1993a;Resnik, 1993b) have proposed the information theoretic approach to measure semantic similarity between two words. Richardson et. al. (1994) has proposed an edge-weight based scheme for Hierarchical Conceptual Graphs (HCG) to measure semantic similarity between words. Efforts (Jiang and Conrath, 1997) have been made to combine both the information content based approach and the graph based approach of predicting semantic similarity. In addition, strategies of using multiple information sources to collect semantic information have also been adopted (Li et al., 2003). Wang and Hirst (2011) have criticized the traditional notions of the depth and density in a lexical taxonomy. However, almost all of the attempts described above have been taken in English based on the representation of WordNet. Das and Bandopadhaya (2010) have proposed a SemanticNet in Bangla, where the relations are based on human pragmatics.

Construction of the Proposed Lexicon
We have taken the Samsad Samarthasabdokoshby Ashok Mukhopadhyay(2005) as the basis for our proposed lexical representation in Bangla. The book contains 757 main words distributed in 30 different sections. Each section addresses a particular domain such as universe-nature-earth, life-living being-body etc. The main words have their corresponding synonyms and similar or related words. Different groups of words that are associated with a single main word are organized together. Relevant information such as Part-Of-Speech (POS) corresponding to every word and antonyms for adjectives are also mentioned. Two types of cross-references are present: one relates two main words or a single word which is simultaneously synonymous to two different main words and the other denotes multiple occurrences of the same. We have termed them as primary link and secondary link respectively. We have also analysed Bangla corpuses: complete novel and story collection of Rabindranath Tagore, Bankimchandra Chattaopadhayay 2 , collection of Bangla blogs over the internet, Bangla corpus by CIIL 3 Mysore and Anandabazar news corpus 4 and have prepared a list of around 4 lakh distinct words in Bangla with their corpus frequencies.
In order to build-up a semantic relation based lexical representation Bangla; we have constructed a hierarchical conceptual graph based on the above mentioned book. We have also individually processed and stored the distinct general words in the book along with their respective details. Our storage and organization of the database facilitate computational processing of the information and efficient searching to retrieve the details associated with any word. Therefore, it will be a useful resource and tool to other psycholinguistic and NLP studies in Bangla. Given a word, its frequency over the five mentioned corpuses, its association with different categories or sub-categories are collected at a single place so that a user can navigate through the storages with low cognitive load. We have also rated the various types of connections among different levels of the graph and developed a mechanism for predicting semantic similarity measures between words in the proposed lexicon. It supports queries like DETAILS(X) (here X can be any type of node of the hierarchy) and SIMILARITY (WORD1, WORD2). The details of the organizational methodology are described below.
The 30 different sections have been considered as 30 root categories. Each category is a collection of concepts, e.g.
-/sense-perception. 757 main words have been organized under the root categories as sub-categories, which are actually concepts, e.g. /smell.The words (mainly nouns, adjectives, verbal nouns and verbal adjectives) have been distributed into separate clusters attached to the sub-categories and they form the leaves of the hierarchy. There is a common root node as antecedent to all the categories. Corresponding to each sub-category, there are two types of clusters: one contains the exact synonyms and the clusters of the other type contain related words or attributes. The words belonging to the same cluster are synonymous. Every category, sub-category and cluster has distinct identification numbers.

Graphical User Interface
We have also developed a Graphical User Interface based on the lexical representation described above. It can perform two jobs. First, it can be used to find the details about a particular word or category present in the database. A user can provide input in two different ways: directly typing the word or selecting from the list of words of different parts of speeches. For the ease of typing Bangla, we have also provided a Bangla virtual keyboard associated with the GUI. Given a word, the system outputs all the available fields associated with the word. It also provides the name and link of the corresponding sub-category and category so that the user can view details about just by clicking on them. If a word belongs to more than one cluster or part-of speech, the GUI shows all the associated clusters and sub-concepts, concepts. User can also navigate to the subconcept(s) associated by primary link or secondary link with the help of the GUI. Second, given two words as input the GUI also calculates the degree of semantic similarity between them along with their corresponding positions in the lexical representation. The method of obtaining the semantic similarity or relatedness measure has been described in the next section.

Semantic Similarity Measure between Bangla Words
As we have discussed in the above sections, along with relating words semantically, the mental lexicon also assigns a degree of similarity between them. Here, we have proposed a simple graph based semantic similarity measure on our proposed lexicon. We have also verified it with user feedbacks. In our proposed lexicon, the nodes from the top to bottom represent generalized to more specialized concepts. Therefore, the semantic distance or edge weights decrease as one moves down the hierarchy. There are 8 types of direct link in the organization:

Type of link
Link weight ( c is a constant whose value can be adjusted according to the need) 1.
member relation: between a word and its cluster 2. between a cluster and its sub-category 3. between a cluster and its sub-sub-category (if present) 4. is-a relation: between a sub-sub-category and its subcategory (if present)

5.
is-a relation: between a sub-category and its root category.
6. between a category and the root.

7.
primary_link: between a word and a sub-category (according to the representation, this distance is greater than a member relation but lesser than the total path length between word and its sub-category) 8.
secondary_link: between a word and a sub-category (this distance is greater than the distance between a subcategory and its category)

TABLE 2-Edge-weight distributions
We have assumed that the all the nodes at a particular level are equal in weight. The semantic distance between any pair of words is measured by the shortest path distance between them: Here, is a constant signifying the scale of measurement. We have taken and , so that a pair of synonyms has a score of 10 out of 10.Therefore, from table 2and equation (1), the semantic similarity values between different types of word pairs are as shown in table 3.
In order to verify whether the proposed approach to measure semantic similarity or relatedness between a pair of words can actually represent the degree of similarity as perceived by human cognition, we have carried out a user survey. The details of the study have been described in the next section.

Score (in a scale of 10)
both the words are in same cluster (synonym) /2* = 10 both the words are in same subcategory , but in different clusters 2( = 6.25 both the words are in same category , but different sub-categorys = 3.57 both the words are from different categorys = 2.5 both the words are from different subcategorys, but connected through primary_link = 6.45 both the words are from different subcategorys, but connected by secondary_link = 5.26 Antonym is a special type of relation -1

User Study
Participants: 25 native speakers of Bangla participated in the experiment with age between 23 years to 36 years. All of them hold a graduate degree in their respective fields and 10 have a post graduate degree.
Experiment data selection and procedure: 50 word pairs were selected from the lexical representation. The word pairs were chosen from the six different categories of relations described in table 4 above, except antonyms. Each user was asked to assign a score from 1 to 10 to each of the 50 word pairs based on their degree of semantic relatedness: 1 for the lowest or no connectivity and 10 for the highest connectivity or synonyms.

Result and Discussions
Perceiving semantic similarity or relatedness between a pair of words or concepts denoted by them depends on the cognitive skill, domain or language knowledge and background of the user. Corresponding to each of the six types of words taken for user study, we have calculated both median and mean of user ratings. Mean has been used because of its popularity and common use, but as mean is very sensitive to outlier or extreme values median has also been taken into account. The table 4 below shows the outcomes of the user validation: The figure 3 below demonstrates the results graphically, it can be easily seen that the user ratings and our proposed measure are very close to each other. One interesting point to be noted here is that the overall mean and median of user ratings for category 1 is less than 10. This means synonyms are not always perceived as exactly similar to each other. Spearman's rank correlation 5 of the predicted semantic similarity measure with the median values of user scores corresponding to each of the 50 word pairs is 0.8. To depict the subjectivity of user's perception, we have plotted the median values against our proposed scores (refer to figure 4).  Figure 4 shows few outliers in the dataset that have median values far from the group mean and median (type 1). Another type (type 2) of word pair is of interest as they have significant difference (greater than 1) between mean and median values, which implies that user ratings contain some extreme values. The pairs belonging to each type are: C word pair Type C word pair Type . /flood-/mountain 2 5 ! 2/hail -03 4 /snowfall 1 5 ) * +/planetary system-5"60, /solar system 2 6 0 , 7 /hightide-* 8 /flood 1 5 -* /farm land-3" /crop 2 3 " 3 ./success-. /fame 1, 2 2 9 /naked-:/undresses 1 6 ;! /iceberg-4/pebbles 1, 2 6 < !--=0 1, 2 TABLE 5-List of type 1 and type 2 words. "C" implies Category.
As can be seen from the above table, word-pairs like ( -) demands a certain level of knowledge about the mythology to be perceived as synonyms, therefore, the user scores corresponding to this kind of word pairs also vary from person to person. Again, the similarity for the word pairs () * +/planetary system-5"60, /solar system) and ( -* /farm land-3" /crops) depend on how a user connects the two concepts in her cognition. The type 1 word pairs such as ( 9 /naked-:/undressed) ( ! 2/hail-03 4 /snowfall) and (" 3 ./success-. /fame) have been marked as synonyms or highly similar by the users. These phenomena demonstrate the confusion in distinguishing synonyms and very closely related concepts or words, especially those which are used alternatively in frequent situations. Three pairs belong to both types signifying they have been perceived as very close by most of the users and at the same time have got extreme values from the rest.

Conclusion and perspective
In this paper, we have proposed a hierarchically organized semantic lexicon in Bangla and also a graph based edge-weighting approach to measure the semantic similarity between two words. The similarity measures have been verified using user studies. We have included the frequency of each word over five Bangla corpuses in our lexical structure and also working on associating more details to words such as, their pronunciations, distribution in spoken corpus, word frequency history over time etc. Our proposed lexical structure contains only relations based on semantic association; we plan to extend the work to incorporate other kinds of relationships such as orthography, phonology and morphology to represent the human cognition more accurately.