An overview of P ortuguese W ord N ets (2016)

Abstract: Semantic relations between words are key to building systems that aim to understand and manipulate language. For English, the “de facto” standard for representing this kind of knowledge is Princeton’s WordNet. Here, we describe the wordnet-like resources currently available for Portuguese: their origins, methods of creation, sizes, and usage restrictions. We start tackling the problem of comparing them, but only in quantitative terms. Finally, we sketch ideas for potential collaboration between some of the projects that produce Portuguese wordnets.

## introduction semantic relations are a key aspect when developing computer programs capable of handling language -they establish (labeled) associations between words and can be integrated into lexicalsemantic knowledge bases. available since the beginning of the 1990s, princeton's wordnet @xcite , henceforth pwn, is a paradigmatic lexical resource. originally created for english, its model is now a "de facto" standard, due to its wide use in applications and its adaptation to different languages. for portuguese, the first resource of this kind, wordnet.pt @xcite , was announced in 2001 but, unlike pwn, was never free to use. this meant that, in practice, there was still no open portuguese wordnet. in parallel, a few alternatives to the wordnet model arose, some of which were compared in @xcite . but if those alternatives proved themselves useful for some tasks, they were not enough to enable all of the standard uses of a wordnet in natural language processing (nlp), including similarity computation or word sense disambiguation. as the need for a portuguese wordnet was keenly felt, in the early 2010s, several projects sprung up aiming to develop free portuguese wordnets. we describe some of those wordnets, while indicating where they were created, their construction process, their availability and, when possible, their size. we recall the wordnet model, its adaptation to other languages, and how these adaptations may be expanded through content alignment. then, we describe the portuguese wordnets we are aware of, alternative lexical-semantic resources, and go on to focus on the open wordnets. after that, we briefly compare the previous along a set of relevant features for processing portuguese. then, we suggest work leveraging what is already planned for these wordnets, as well as some ideas for collaboration. knowing where we are in terms of our wordnets is an essential first step in establishing lexical resources, which are vital to the computational processing of the portuguese language. 1 2 wordnet and alternatives lexical knowledge bases are organized repositories of lexical items, usually including information about the possible meanings of words, relations between them, definitions, and phrases that exemplify their use. the princeton wordnet model, with english as its target language, is probably the most popular representative of this type of lexical knowledge base. its flexibility has led not only to its growing use by the nlp community, but also to the adaptation of the model to other languages. pwn was created manually in the early 1990s and has been updated several times since then. initially based on psycholinguistic principles, it combines traditional lexicographic information, similar to that in a dictionary, with an appropriate organization for computational use, which facilitates its application as a basis for lexical-semantic knowledge. like a thesaurus, pwn is organized in groups of synonymous lexical items, called synsets, which can be seen as the possible lexicalizations for the concepts in the language. besides synonymy, inherent to synsets, pwn covers other types of semantic relation between synsets. for example, hypernymy -a concept is a generalization of another -or meronymy -a concept is a part of another. in addition, each synset has a part-ofspeech (noun, verb, adjective or adverb); a gloss, similar to a definition in a dictionary; and it may still have phrases that illustrate its use. the inclusion of a lexical item in a synset indicates a sense of that item. both its free availability and the flexibility of its model were crucial to the success and widespread use of pwn. this made it possible to integrate pwn into a large number of nlp or knowledge management projects, making it virtually the standard model of a lexical resource for several languages. the popularity of the pwn knowledge base model led to the creation of the global wordnet association (gwa), foot_2 a non-commercial organization that provides a platform for discussion, sharing and linking the wordnets of the world. ## open portuguese wordnets open wordnets for portuguese finaly appeared in the early 2010s. they were created by automatic or semi-automatic means and all assume that lexical-semantic resources must be open-source to be really useful to the community. we present four wordnets that fall in this category. ## comparing open wordnets we attempted a superficial comparison of their latest versions, that should not be seen as more than a purely quantitative tabling. we have not tried to compare the consistency nor the usefulness of the contents of the various portuguese wordnets. on the number of covered lexical items, onto.pt stands out for including more than three times more lexical items than the second largest wordnet, openwn-pt. this confirms that a fully automatic construction approach leads to a larger on the number of word senses, synsets and relation instances, onto.pt also stands out from the rest. but it should be noted that there is an intrinsic trade-off between the size of a wordnet and the accuracy and usefulness of the resource under scrutiny. one of the difficulties in developing a wordnet is precisely to decide, on the one hand, if two words are to be regarded as synonymous and thus placed within the same synset and, on the other hand, which words should be in different synsets. these are typical lexicography challenges to which there is probably no final unique answer. but there seems to be a consensus that a very large number of synsets is a sign of "noise" in the process of grouping words and/or in the discrimination process. correction/accuracy is undoubtedly one of the bottlenecks of building wordnets. if, on the one hand, size and coverage are a quantitative comparison, which is relatively simple, the same cannot be said about the quality assessment. the english pwn, built manually, may even reflect questionable decisions, but does not contain "errors" as such, as we are using it as a baseline for comparison. as for the wordnets built automat-ically, or semi-automatically, for languages other than english, quality assessment will always be an issue, since there is no golden reference available -this is precisely what they want to become. from this perspective, resources that rely on human labor have an advantage, although we do not know exactly how this advantage can or should be measured. an alignment with pwn may be important for obtaining additional knowledge,mostly from other resources aligned with it. in addition to relation inheritance, an alignment allows access to knowledge of other extensions of pwn, such as wordnet-domains, sentiwordnet or tem-powordnet. on the other hand, a blind alignment does not consider that different languages represent different socio-cultural realities, do not cover the same part of the lexicon and, even where they seem to be common, several concepts are lexicalized differently @xcite . both wn.pt and onto.pt cover a wide range of relation types, some not typically present in wordnets. we recall that, for onto.pt, these extracted was possible due to the regularities in dictionary definitions. additional comparison tables, with more details, are provided in @xcite . ## building on open wordnets we presented and compared various wordnets that currently exist for portuguese. among them, four are freely available. this is a vast improvement. until recently, the situation was very different: one synset base (tep), no definitions, was freely available; one wordnet (mwn.pt) could be purchased; and another could only be explored on-line (wn.pt). the creation of these wordnets followed different approaches, from completely manual labour, through translation-based approaches with more or less manual labour, to an approach in which the whole structure is populated automatically. we hope to have shown that, currently, it makes no sense to regret that there is no portuguese wordnet. in fact, the use of a wordnet in a project targeting portuguese is becoming less of a problem of finding a work-around solution, and increasingly more one of choosing the most suitable within the available alternatives. this selection should consider, among other things, the need to align with other wordnets, the error tolerance, the coverage needs -both with regards to the lexical items and to relationships between them -and even the available budget. since each wordnet has distinct characteristics, one should not discard the use of more than one wordnet in the same project. it is sensible to ask whether all these alternatives make sense or if it would be preferable to focus on a single effort to build a single portuguese wordnet, trying to harness the strong points of each of the projects described. the authors of this article, responsible for onto.pt, openwn-pt and pulo, believe that there are advantages both on converging into a single wordnet and on keeping separate projects. thus, in the short term, the development of each wordnet will remain the responsability of its original team, but there will be a closer monitoring of each other's work. the idea is that each project may reuse what is done by the others, this way minimizing duplicate work, but without losing sight of its specific goals. in a near future, onto.pt will become a fuzzy wordnet, based on the redundancy across several portuguese computational lexical resources, including the other open wordnets, whose further updates will be welcome by this new initiative. following eco, confidence degrees will be assigned to each decision taken, including the membership of words to synsets or the attachment of relations to synsets -for recent work the automatic extraction of fuzzy synsets from seven open lexical resources for portuguese, check (gonc ¸alo oliveira and @xcite . this will enable the users to, depending on their purpose, set a cutpoint to select between a larger but less reliable resource or a smaller one with fewer issues. openwn-pt has been experimenting with the definitions and examples of pwn in portuguese, produced by pulo and hopes to experiment also with the the extra relations of causation, purpose, location or manner, produced by onto.pt. more generally the open wordnets will consider the integration of the contents of each other, and/or ways of replicating their approaches to enrichment of their own resources. ## conclusions we presented a collection of portuguese wordnets, with an emphasis on the open initiatives, and their shallow comparison. while none feels as mature as princeton wordnet, some have already been used in applications. joint efforts, as we started doing and hope to do more, seem to be the only way of making progress in this hard problem. clearly, the envisaged applications will lead to slightly different strong points in our resources, to provide wordnets that are open, large coverage and as reliable as possible.