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<Paper uid="W04-2214">
  <Title>Revising the WORDNET DOMAINS Hierarchy: semantics, coverage and balancing</Title>
  <Section position="3" start_page="0" end_page="0" type="metho">
    <SectionTitle>
2 The WordNet Domains Hierarchy
</SectionTitle>
    <Paragraph position="0"> The first version of the WDH was composed of 164 domain labels selected starting from the subject field codes used in current dictionaries, and the subject codes contained in the Dewey Decimal Classification (DDC), a general knowledge organization tool which is the most widely used taxonomy for library organization purposes.</Paragraph>
    <Paragraph position="1"> Domain labels were organized in five main trees, reaching a maximum depth of four. Figure 1 shows a fragment of one of the five main trees in the  Domain labels were initially conceived to be application-oriented, that is, they have been integrated in WordNet with the main purpose of allowing the categorization of word senses and to provide useful information during the disambiguation process.</Paragraph>
    <Paragraph position="2"> The second level of WDH, where the so-called Basic Domains are represented, includes labels such as ART, SPORT, RELIGION and HISTORY, while in the third level a degree of major specialization is reproduced, and domains, like for example, DRAWING, PAINTING, TENNIS, VOLLEYBALL, and ARCHAEOLOGY can be found. For NLP tasks, the set of Basic Domains has proved to possess a suitable level of abstraction and granularity.</Paragraph>
    <Paragraph position="3"> Although the first version of WDH found many applications in different scenarios, it presented some problems. First, the domain labels did not have a defined semantics. The content of the labels could be suggested by the lexical meaning of their name, but there was no explicit indication about their intended interpretation.</Paragraph>
    <Paragraph position="4"> Second, it was not clear whether the Basic Domains met certain requirements such as knowledge coverage and balancing. In fact, the Basic Domains are supposed to possess a comparable degree of granularity and, at the same time, to cover all human knowledge. However, they did not always posses such characteristics. For instance VETERINARY was put at the same level as ECONOMY, although these two domains obviously do not posses the same level of granularity.</Paragraph>
    <Paragraph position="5"> Moreover not all branches of human knowledge were represented (see for instance the HOME domain).</Paragraph>
    <Paragraph position="6"> The purpose of the work presented here was, therefore, to find a solution for such problems, in order to improve the applicability of WDH in a wider range of fields. The solution we propose is crucially based on the Dewey Decimal Classification (edition 21), which has been used as a reference point for defining a clear semantics, preventing overlapping among domains, and assessing the Basic Domains coverage and granularity issues.</Paragraph>
    <Section position="1" start_page="0" end_page="0" type="sub_section">
      <SectionTitle>
2.1 The Dewey Decimal Classification (DDC)
The Dewey Decimal Classification (DDC) system
</SectionTitle>
      <Paragraph position="0"> (Mitchell et al. 1996) is the most widely used taxonomy for library classification purposes providing a logical system for the organization of every item of knowledge through well-defined subject codes hierarchically organized. The semantics of each subject code is determined by a numeric code, a short lexical description associated to it, and by the hierarchical relations with the other subject codes. Another characteristic of the DDC is that a handbook is available explaining how texts should be classified under subject codes.</Paragraph>
      <Paragraph position="1"> The DDC is not just for organizing book collections; it has also been licensed for cataloguing internet resources (see for example BUBL http://bubl.ac.uk/link/) and it was conceived to accommodate the expansion and evolution of the body of human knowledge.</Paragraph>
      <Paragraph position="2"> The DDC hierarchy is arranged by disciplines (or fields of study), and this entails that a subject may appear in more than one discipline, depending on the aspect of the topic discussed.</Paragraph>
      <Paragraph position="3"> The DDC hierarchical structure allows a topic to be defined as part of the broader topic above it, and that determines the meaning of the class and its relation to other classes. At the broadest level, called Main Classes (or First summary), the DDC is composed of ten mutually exclusive main classes, which together cover the entire world of knowledge. Each main class is sub-divided into ten divisions, (the Hundred Divisions, or Second Summary) and each division is split into ten sections (the Thousand Section, also called Third Summary).</Paragraph>
      <Paragraph position="4"> Each category in the DDC is represented by a numeric code as the example below shows.</Paragraph>
      <Paragraph position="5">  The first digit of the numbers indicates the main class, (700 is used for all Arts) the second digit indicates the hundred division, (730 corresponds to Plastic arts, 740 to Drawing, 750 to Painting) and the third digit indicates the section (736 represents Carving, 738 Ceramic arts, 739 Art metalwork).</Paragraph>
      <Paragraph position="6"> Moreover, almost all sub-classes are further subdivided. A decimal point follows the third digit until the degree of specification needed (736.23 Diamonds, 736.25 Sapphires).</Paragraph>
    </Section>
  </Section>
  <Section position="4" start_page="0" end_page="0" type="metho">
    <SectionTitle>
3 The Revision of the WDH
</SectionTitle>
    <Paragraph position="0"> The revision of the first version of the WDH aimed at satisfying the following properties and characteristics: o semantics: each WDH label should have an explicit semantics and should be unambiguously identified; o disjunction: the interpretation of all WDH labels should not overlap; o basic coverage: all human knowledge should be covered by the Basic Domains; o basic balancing: most Basic Domains should have a comparable degree of granularity. In the following sections we are going to show how a systematic mapping between WDH and DDC can be used to enforce each of the above characteristics.</Paragraph>
    <Section position="1" start_page="0" end_page="0" type="sub_section">
      <SectionTitle>
3.1 Semantics
</SectionTitle>
      <Paragraph position="0"> To give the domain labels a clear semantics so that they can be unambiguously identified and interpreted, we decided to associate each domain label to one or more DDC codes as shown below in  respective DDC codes In many cases we found a one-to-one mapping between a WDH label and a DDC code (e.g. PAINTING mapped onto 750 or CINEMA onto 791.43). When one-to-one mappings were not found, artificial DDC codes were created. An artificial code, represented within square brackets, is created with reference to various DDC codes or parts of them. To describe artificial nodes, certain  conventions have been adopted.</Paragraph>
      <Paragraph position="1"> (i) A series of non-consecutive codes is listed separated by a comma (see DANCE).</Paragraph>
      <Paragraph position="2"> (ii) A series of consecutive codes is indicated by a range. For instance, the series [731, 732, 733, 734, 735] is abbreviated as [731:735] (see SCULPTURE). (iii) A part of a tree is represented as the difference  between a tree and one or more of its subtrees, where the tree and the subtrees are identified by their roots (see DRAWING).</Paragraph>
      <Paragraph position="3"> (iv) The square brackets should be interpreted as meaning &amp;quot;the generalities&amp;quot; of the composition of codes contained in the brackets. So, for instance, [731:735] should be interpreted as the generalities of the codes going from 731 to 735. In the original DDC, generalities are identified by the 0 decimal. For instance, the code 700 refers to the generalities of the codes from 710 to 790.</Paragraph>
      <Paragraph position="4"> To establish a mapping between labels and codes we exploited the names of the DDC categories and their description in the DDC manual. This worked pretty well in most cases, but there are some exceptions. Take for instance the TOURISM domain. Apparently tourism does not occur as a category in the DDC. On a closer inspection it came out that the categories which are most clearly related to tourism are 910.202:World travel guides and 910.4:Accounts of travel.</Paragraph>
      <Paragraph position="5"> Note that a WDH domain can be mapped onto codes included in different DDC main classes, i.e. disciplines. For example ARTISANSHIP (745.5:Handicrafts, 338.642:Small business) maps onto categories located partly under 700:Art and partly under 300:Social Sciences. The same happens with SEXUALITY, a domain that following the DDC is studied by many different disciplines, e.g. philosophy, medicine, psychology, body care. As a consequence of the systematic specification of the semantics of the WDH domains, some of them have been re-labeled with regard to the previous version of the hierarchy. For instance, the domain BOTANY has been changed to PLANTS, ZOOLOGY to ANIMALS, and ALIMENTATION to FOOD.</Paragraph>
      <Paragraph position="6"> This change of focus from the name of the discipline to the name of the object of the discipline is not only in compliance with the new edition of the DDC, but it also reflects current and international usage (see, for example, Google categories). In some cases the change of the domain name comes along with a change of its intended interpretation. For instance, we have decided to enlarge the semantics of the domain ZOOTECHNICS and to call it ANIMAL HUSBANDRY, a more generic domain which was missing in the previous hierarchy.</Paragraph>
      <Paragraph position="7"> In most cases the hierarchical relations between the WDH domains are the same as the relations holding between the corresponding DDC codes: MUSIC is more specific than ART in the same way as 780:Music is more specific than 700:The Arts. To reinforce the hierarchical parallelism between the WDH and the DCC, we re-located some domains with regard to the previous WDH hierarchy. For example, OCCULTISM, which was placed under RELIGION in the old hierarchy, has been moved under the newly created domain PARANORMAL. Also, TOPOGRAPHY, previously placed under ASTRONOMY, has now been moved under GEOGRAPHY.</Paragraph>
      <Paragraph position="8"> In a few cases however we did not respect the hierarchical relations specified by the DDC, as in the case of the ARCHITECTURE domain shown in  However, whereas the 710 code is sibling of 720 in the DDC, TOWN PLANNING is child of ARCHITECTURE in WDH. Also, ARCHITECTURE and TOWN PLANNING should be under ART according to the DDC, but they have been placed under</Paragraph>
    </Section>
  </Section>
  <Section position="5" start_page="0" end_page="0" type="metho">
    <SectionTitle>
APPLIED SCIENCE in WDH.
</SectionTitle>
    <Paragraph position="0"/>
    <Section position="1" start_page="0" end_page="0" type="sub_section">
      <SectionTitle>
3.2 Disjunction
</SectionTitle>
      <Paragraph position="0"> This property requires that no DDC code is associated to more than one WDH label. In only one case this requirement has not been met.</Paragraph>
      <Paragraph position="1"> Apparently, the DDC does not distinguish between the disciplines of Sociology and Anthropology, and reserves the codes that go from 301 to 307 to both of them. Although these two disciplines are strictly connected, it seems to us that in the current practice they are considered as distinct. So the WDH contains two distinct domains for SOCIOLOGY and ANTHROPOLOGY, which partially overlap because they both map onto the same DDC codes 301:307.</Paragraph>
    </Section>
    <Section position="2" start_page="0" end_page="0" type="sub_section">
      <SectionTitle>
3.3 Basic Coverage
</SectionTitle>
      <Paragraph position="0"> The term basic coverage refers to the ideal requirement that all human knowledge be covered by the totality of the Basic Domains (i.e. the domains composing the second level of WDH).</Paragraph>
      <Paragraph position="1"> Also in this case, we used the DDC as a gold standard to measure the coverage of WDH. Given the fact that the DDC has been used for more than a century to classify books and written documents all over the world, we can assume that the DDC guarantees a complete representation of all branches of knowledge. So the basic coverage has been manually checked by verifying that all (or almost all) the DDC categories can be assigned to at least one Basic Domain.</Paragraph>
      <Paragraph position="2"> From a practical point of view, it would be very complicated to check all the thousands of codes contained in the DDC. Thus, our check relied on two assumptions. First, when the Basic Domains are taken as a stand alone set, the semantics of a Basic Domain is given by its specific code together with the codes of its subdomains. Second, once a DDC code is covered by a Basic Domain, inductively, all the more specific categories are covered as well. These assumptions allowed us to actually check only the topmost DDC codes. For example, let's take the 300 main class of the DDC. Table 3 below shows that all the sub-codes of the 300 class are covered by one or more domains.</Paragraph>
      <Paragraph position="3"> In order to improve the overall WDH coverage, 5 completely new domains have been introduced (the first three are Basic): PARANORMAL, HOME, HEALTH, FINANCE and GRAPHIC ARTS.</Paragraph>
      <Paragraph position="4">  We can now assume that the domain-coverage of the new version of WDH is almost equivalent to that of the DDC, thus ensuring the complete representation of all branches of knowledge.</Paragraph>
      <Paragraph position="5"> The new WDH allowed us to fix a number of synset classifications that were unsatisfactory in the previous version of WORDNET DOMAINS. For instance, in the first version of WORDNET DOMAINS the English/Italian synset {microwave oven, microwave}/{forno a microonde, microonde} was annotated with the FURNITURE domain, while the synset {detergent}/{detersivo} was annotated with FACTOTUM (i.e. no specific domain) as no better solution was available. The new WDH hierarchy allows for a more appropriate classification of both synsets within the new HOME domain.</Paragraph>
      <Paragraph position="6"> A few DDC codes are not covered by the new list of domains either. These are the codes under the 000:Generalities class which includes disciplines such as 010:Bibliography, 020:Library &amp; information sciences, 030:Encyclopedic works, 080:General collections. This section has been specifically created for cataloguing general and encyclopedic works and collections. So it is a idiosyncratic category which is not based on subject but on the genre of texts.</Paragraph>
      <Paragraph position="7"> Another set of codes which remains not covered by WDH are those going from 420 to 490 and from 810 to 890. These DDC codes are devoted to specific languages and literatures of different countries, for example, 430:Germanic Languages, 440:Romance Languages, 810:American Literature in English, etc. These codes are undoubtedly relevant for the classification of books, but are not compatible with the rationale of WDH, which is meant to be a language-independent resource.</Paragraph>
    </Section>
    <Section position="3" start_page="0" end_page="0" type="sub_section">
      <SectionTitle>
3.4 Basic Balancing
</SectionTitle>
      <Paragraph position="0"> The requirement about basic balancing is meant to assure that all Basic Domains have a comparable degree of granularity.</Paragraph>
      <Paragraph position="1"> Defining a granularity metrics for domains is a complex issue, for which only a tentative solution is provided here. At a first glance, three aspects could be taken into consideration: the number of publications about a domain, the number of sub-codes in the DDC, and the relevance of a domain in the social life.</Paragraph>
      <Paragraph position="2"> As a first attempt, balancing could be evaluated referring to the number of publications classified under each Basic Domain. In fact, data are available about the number of texts classified under each of the DDC codes. Unfortunately, the number of books published under a certain category may not be indicative of its social relevance: very specialized domains may include a high number of publications, which however circulate in a restricted circle, with low social impact. For example, the number of texts classified in the History domain turns out to be more then ten times the number of texts catalogued under the Computer Science domain. However, if one looks at the number of HTML pages available on the Internet, or the number of magazines sold in a newspaper stand, or the number of terms used in everyday life, one cannot maintain that History is ten times more relevant than Computer Science.</Paragraph>
      <Paragraph position="3"> Another approach for evaluating the granularity of domains could be to take into account the number of DDC sub-codes corresponding to each Basic Domain. Unfortunately, also this approach gives results which are far from being satisfactory. The fact that a discipline has many subdivisions seems not to be clearly correlated with its relevance. For instance in the DDC manual (version 21) 105 pages can be put in correspondence with the ENGINEERING domain, whereas only 26 correspond to SPORT. It should also be said that there is no correlation between the number of publications and the number of sub-categories in the DDC. For instance, ARCHITECTURE has a great number of publications classified under it, but on the contrary, the number of sub-categories in the DDC is very limited.</Paragraph>
      <Paragraph position="4"> The third criterion to evaluate the granularity of domains is their social relevance, which seems not to be captured adequately by the previous two criteria. Of course, social relevance is very difficult to evaluate. We tentatively took into consideration the organization of Internet hierarchies such as the Google and Yahoo directories, which seem to be closer than the DDC to represent the current social relevance of certain domains. See for instance the huge number of HTML pages classified in Google under the topic Television Programs. Of course Internet is only a partial view of the organization of human knowledge, so we cannot simply rely on the Internet to evaluate the granularity of the domains.</Paragraph>
      <Paragraph position="5"> None of the approaches analyzed so far seems to fit our needs. Thus we took into consideration a fourth criterion, which is based on the DDC as well. Instead of counting the number of subdivisions under a certain DDC code, we measured the depth of the code from the top of the hierarchy. For instance we can say that 700:Art has depth 1, 780:Music has depth 2, 782:Vocal Music has depth 3, and so on. We make the assumption that two DDC codes with the same depth have the same granularity. For instance we assume that 782:Vocal Music and 382:Foreign Trade have the same granularity (both have depth 3).</Paragraph>
      <Paragraph position="6"> In order to evaluate the granularity of the Basic Domains against the DDC, we can compare WDH labels and DDC codes with the same depth. Given that the Basic Domains have depth 2, we should compare them to the so called Hundred Divisions (000, 010, 020, 030, ..., 100, 110, 120, etc.).</Paragraph>
      <Paragraph position="7"> Summing up, we will say that the Basic Domains are balanced if they can all be mapped onto the Hundred Divisions. Also, in the comparison we should take into account that the Basic Domains are 45, whereas the Hundred Divisions are 100. So, we expect that in the average, one Basic Domain maps onto two Hundred Divisions with a small degree of variance with respect to the average.</Paragraph>
      <Paragraph position="8"> What we have obtained from the analysis of the new WDH is the following: out of 45 Basic Domains o 4 domains map onto a Main Class (depth 1) o 18 domains are mapped at the Hundred Divisions level (depth 2) o 6 domains are mapped at different DDC levels, with the majority of DDC codes at depth 2 o 17 domains map onto subdivisions of depth 3 and 4.</Paragraph>
      <Paragraph position="9"> As for the average number of DDC codes covered by each Basic Domain, the variance is quite high. Certain Basic Domains cover a big number of codes from the Hundred Divisions. For instance HISTORY, and ART cover 6 codes each.</Paragraph>
      <Paragraph position="10"> Instead, in most cases, one Basic Domain covers only one DDC code (e.g. LAW and 340:Law).</Paragraph>
      <Paragraph position="11"> The evaluation of the granularity of the Basic Domains according to the proposed criterion can be considered satisfactory even if the results diverge somewhat from what expected in principle. To explain this partial divergence in the granularity of domains, one should take into consideration that the DDC has been created relying heavily on the academic organization of knowledge disciplines. On the other side, in the practical WDH reorganization process we tried to balance somehow this discipline-oriented approach, by taking into account also the social relevance of domains. This has been done by relying on the organization of Internet directories and on our personal intuitions.</Paragraph>
      <Paragraph position="12"> Such an approach led us to put at the Basic level WDH labels corresponding to DDC codes with depth higher than 2 (more specific than the Hundreds Divisions). See for instance the positioning of RADIO+TV, FOOD, HEALTH, and ENVIRONMENT at the Basic level, even if they correspond to DDC codes of level 3 and 4.</Paragraph>
      <Paragraph position="13"> Instead, ANIMALS and PLANTS were not Basic in the previous version of WDH, but have been promoted to the Basic level in accordance with the granularity level they have in the DDC.</Paragraph>
      <Paragraph position="14"> Other domain labels have been placed at a lower level then expected with reference to the DDC. For instance PHILOSOPHY, ART, RELIGION, and LITERATURE have been put at the Basic Level, even if they correspond to DDC codes belonging to the Main Classes (depth 1). On the other side ASTROLOGY, ARCHAEOLOGY, BODY CARE, and VETERINARY which were Basic in the previous version of the WDH, have been demoted at a lower level in accordance with the granularity they have in the DDC. Only in one case this process of demotion has led to the elimination of a subdomain, that is TEXTILE.</Paragraph>
    </Section>
  </Section>
  <Section position="6" start_page="0" end_page="0" type="metho">
    <SectionTitle>
4 Conclusions
</SectionTitle>
    <Paragraph position="0"> In this paper we described the revision of the WORDNET DOMAINS Hierarchy (WDH), with the aim of providing it with a clear semantics, and evaluating the coverage and balancing of a subset of the WDH, called Basic Domains. This has been done mostly by relying on the information available in the Dewy Decimal Classification (DDC). A semantics has been provided to the WDH labels by defining one or more pointers to DDC codes. The coverage of the Basic Domains has been evaluated by checking that each DDC code is covered by at least one Basic Domain.</Paragraph>
    <Paragraph position="1"> Finally, balancing has been evaluated mostly by comparing the granularity of the Basic Domains with the granularity of a subset of the DDC called the Hundred Divisions. Balancing is the aspect of the Basic Domains which diverges more clearly from the DDC. This is explained by the fact that we took in higher consideration the social relevance of domains.</Paragraph>
    <Paragraph position="2"> We think that the new version of the WDH is better suited to act as a useful language-independent resource in the fields of computational lexicography, corpus building, and various NLP applications.</Paragraph>
  </Section>
  <Section position="7" start_page="0" end_page="0" type="metho">
    <SectionTitle>
5 Acknowledgements
</SectionTitle>
    <Paragraph position="0"> Thanks to Alfio Gliozzo for his useful comments and suggestions about how to improve the</Paragraph>
  </Section>
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