Relationships in the Dewey Decimal Classification System
Joan S. Mitchell
OCLC Forest Press
Abstract
The Dewey Decimal Classification (DDC) system is a general knowledge organization tool used worldwide. The system features well-defined categories, well-developed hierarchies, meaningful notation, and a rich network of relationships. The nature of relationships in Dewey is richer than is apparent from a linear view of the system. The DDC accommodates the three basic thesaural relationships in the notational and structural hierarchy; through notes in the schedules, tables, and Manual; and in entries in the Relative Index. Derived relationships are shown through the synthesized notation, or through abridgment leading to broader classification. Options and translations provide alternative relationships. Additional relationships may be displayed through mapping of other thesauri.
The Dewey Decimal Classification (DDC) system is a general knowledge organization tool used worldwide in 135 countries. The DDC is owned by OCLC Forest Press, and is published in full (Dewey, 1996) and abridged (Dewey, 1997) print editions, and an electronic (Dewey for Windows, 1996) version. A Web version is under development. The DDC has also been translated into over thirty languages.
The DDC is developed and updated continuously in the Dewey editorial office at the Library of Congress under an agreement between OCLC Forest Press and the Library of Congress. The system features well-defined categories, well-developed hierarchies, meaningful notation, and a rich network of relationships. In this chapter, we will focus on the relationships found or derived from the DDC, and the relationships that may be mapped to the DDC. The DDC accommodates the three basic thesaural relationships: hierarchical, equivalence, and associative relationships (National Information Standards Organization, 1994). The various relationships are displayed in the notational and structural hierarchy; through notes in the schedules, tables, and Manual; and in entries in the Relative Index. Those that can be derived are shown through the synthesized notation, or through abridgment leading to broader classification. Options and translations provide alternative relationships. Additional relationships may be displayed through mapping of other thesauri.
1.0 Brief Introduction to the DDC
In order to understand the relationships within, derived from, or mapped to the DDC, it is important to have a basic understanding of the design of the system. The DDC is divided into ten main classes, which together cover the entire world of knowledge. Each main class is further divided into ten divisions and each division into ten sections (not all the numbers for the divisions and sections have been used). Arabic numerals are used to represent each class in the DDC. A decimal point follows the third digit, after which division by ten continues to the specific degree of classification needed.
Below are the ten main classes with their popular descriptions. The first digit in each three-digit number represents the main class. For example, 500 represents science.
Dewey Decimal Classification
Ten Main Classes
000 Computers, Information & General Reference
100 Philosophy & Psychology
200 Religion
300 Social Sciences
400 Language
500 Science
600 Technology
700 Arts & Recreation
800 Literature
The second digit indicates the division. For example, 500 is used for general works on the sciences, 510 for mathematics, 520 for astronomy, 530 for physics.
500 Science
510 Mathematics
520 Astronomy
530 Physics
540 Chemistry
550 Earth Sciences & Geology
560 Fossils & Prehistoric Life
570 Biology & Life Sciences
580 Plants (Botany)
590 Animals (Zoology)
The third digit in each three-digit number indicates the section. For example, 510 is used for general works on mathematics; the specific branches of the discipline are divided among 511-519.
510 Mathematics
519 Probabilities and applied mathematics
The DDC is developed on the basis of literary warrant; that is, a threshold of published literature determines the need for category creation. The system includes over 25,000 entries in the core disciplinary outline of knowledge in 000-999. These are called the schedules. Supplementing the schedules are seven auxiliary tables of notation that may be added to numbers in the schedules to show form, place, time, people, language, genre, and other aspects. The schedules also are extended by internal tables, and by add instructions from other parts of the schedules.
Each entry in the schedules and tables consists of a class number and caption. Some entries have special labels or captions that indicate a structural hierarchy different from the notational hierarchy. Many entries have one or more notes that explain the hierarchical, equivalence, and associative relationships between that class and other classes in the DDC. The entries may also contain instructions for extending the notation to include additional aspects through the use of internal tables, notation from elsewhere in the schedules, or the auxiliary tables.
2.0 Relationships in the Schedules and Tables
2.1 The Hierarchical Relationship
The hierarchical relationship in the DDC is expressed through notation and structure. In the DDC, all topics (aside from the ten main classes) are part of the broader topics above them. Certain notes (definition, scope, relocation, class-here, class-elsewhere, see-reference, preference, and option notes) have hierarchical force for a class and all its subordinate classes. There is a strong inheritance principle at work in the hierarchical relationship, whether that relationship is described through the notation or through other devices in the DDC.
2.1.1 Notational Hierarchy
In many places throughout the DDC, the hierarchy is expressed by length of notation. Numbers at any given level are usually subordinate to a class whose notation is one digit shorter, coordinate with a class whose notation has the same number of significant digits, and superordinate to a class with numbers one or more digits longer. The following example shows the notational hierarchy for several kinds of racket games.
796 Athletic and outdoor sports and games
796.3 Ball games
796.34 Racket games
796.342 Tennis (Lawn tennis)
796.343 Squash
796.345 Badminton
796.346 Table tennis
796.347 Lacrosse
2.1.2 Structural Hierarchy
Sometimes, other devices must be used to express the hierarchy when it is not possible or desirable to do so through the notation. Special types of headings, notes, and entries indicate relationships among topics that violate notational hierarchy.
Dual and multiple headings are used to indicate when a topic and the main subconcept share the same number and hierarchical structure:
972 Middle America Mexico
636.2 Ruminants and Camelidae Bovidae Cattle
Several types of notes are used to indicate the hierarchical relationship. When part of a topic is not included in the direct subdivisions of the number, a see reference leads to its location:
512.02 Abstract algebra
For subdivisions of abstract algebra, see 512.2-512.5
A centered entry indicates and relates structurally a span of numbers that represents a single concept for which there is no specific hierarchical notation available. A symbol > in the number column identifies a centered entry, and it is so called because the span appears in the print edition of the DDC in the center of the page rather than in the number column. For example, the subdivisions of 512.02 Abstract algebra are not notational subdivisions of 512.02, but are located elsewhere under a centered entry:
> 512.2-512.5 Subdivisions of abstract algebra
Class comprehensive works in 512.02
2.1.3 Generic, Whole-Part, and Instance Relationships
The DDC supports generic, whole-part, and instance relationships through the notational and structural hierarchy. A simple example of the generic relationship expressed through the notational hierarchy is the development for the plant subclass 583.3 Ranunculidae:
583.3 Ranunculidae
583.34 Ranunculales (Ranales)
583.35 Papaverales (Rhoeadales)
583.36 Sarraceniales
Ranunculidae are part of 583 Magnoliopsida (Dicotyledons), which in turn are a kind of
Angiospermae (Flowering plants). The generic relationship between Magnoliopsida and Angiospermae is expressed through the structural hierarchy instead of the notational hierarchy:
>583-584 Angiospermae (Flowering plants)
Class comprehensive works in 580
The whole-part relationship is expressed through the notation and structure. Categories of wholes and their parts are found throughout the DDC. For example, here is the discipline of mathematics and its major parts:
510 Mathematics
511 General principles
512 Algebra, number theory
513 Arithmetic
514 Topology
515 Analysis
516 Geometry
519 Probabilities and applied mathematics
In the following example from medicine, the mouth and its parts are displayed in the notational hierarchy:
611.31 Mouth
611.313 Tongue
611.314 Teeth
611.315 Palate
611.316 Salivary glands
611.317 Lips
611.318 Cheeks
In Table 2, the parts of Norway are developed on coordinate notation; the whole-part relationship is revealed through the structural hierarchy:
--481 Norway
> --482-484 Divisions of Norway
--482 Southeastern Norway (Østlandet)
--483 Southwestern Norway (Sørlandet and Vestlandet)
--484 Central and northern Norway (Trøndelag and Nord-Norge)
The instance relationship is sometimes displayed explicitly in the notational and structural hierarchy, and sometimes accommodated through optional arrangements. For example, specific oceans and seas are listed on coordinate notation under the Table 2 notation for oceans and seas:
--162 Oceans and seas
For Atlantic Ocean, see T2--163;
for Pacific Ocean, see T2--164;
for Indian Ocean, see T2--165;
for Antarctic waters, see T2--167. . .
At 005.133 Specific programming languages, the addition of an optional arrangement is suggested to identify specific programming languages by name:
005.133 Specific programming languages
Arrange alphabetically by name of programming language, e.g., C++
2.1.4 Polyhierarchical Relationships
Sometimes, the same concept can be considered as a member of more than one category. The structural hierarchy also addresses polyhierarchical relationships. For example, the comprehensive number for craters is 551.21, the number for volcanoes. A note leads one to 551.21 for craters from another possible category, 551.44 Depressions and openings. Craters formed by meteorites are potentially members of the general category for craters or the category for meteorites as agents of geological change. The latter has been chosen, and a see reference leads from 551.21 to 551.397 for meteorite craters:
551.21 Volcanoes
Class here comprehensive works on craters
For meteorite craters, see 551.397
551.397 Meterorites
Class here meterorite craters
551.44 Depressions and openings
For craters, see 551.21
2.2 The Equivalence Relationship
One of the basic rules in number building in Dewey is that one cannot add subdivisions for a topic unless the concept approximates the whole of the concept represented by the number. Therefore, the equivalence relationship is important in Dewey, and is supported by a number of different types of headings and notes that identify synonyms, variant and former names, and variant spellings. Antonyms are treated in the same manner as synonyms. Several devices are available to denote an equivalence relationship for narrower or broader concepts.
In headings, the equivalence relationship is indicated by parentheses. The term or phrase in parentheses may be a synonym; an equivalent proper adjective for a people, language, or area; an acronym or the spelled-out version of an acronym (depending on which form is the better known); the vernacular form of a geographic name; or the popular version of a scientific name:
153.8 Will (Volition)
--9635 Gur (Voltaic) languages [Table 6]
538.364 Electron paramagnetic resonance (EPR)
572.86 DNA (Deoxyribonucleic acid)
--49453 Fribourg (Freiburg) [Table 2]
595.799 Apoidea (Bees)
When it is not practical to specify the equivalence relationship within the heading, a variant or former name note is included:
530.475 Diffusion and mass transfer
Variant name for mass transfer: mass transport
641.8654 Cookies
Variant name: biscuits (United Kingdom)
--6751 Democratic Republic of the Congo [Table 2]
Former names: Belgian Congo, Zaire
Several notes are used to define the equivalence relationship. Topics in the note may be equivalent, narrower, or broader in scope; but have been deemed to have the same importance as the concept defined in the heading. In some case, this results in what Aitchison, Gilchrist, and Bawden (1997) call "upward posting," e.g., the equivalence between the city of Bologna
and the province of Bologna in Table 2.
746.7 Rugs
Class here carpets
--4541 Bologna province [Table 2]
Class here Bologna
--0832 Infants [Table 1]
Children from birth through age two
2.3 The Associative Relationship
Two kinds of notes indicate the associative relationship in the Dewey schedules and tables: the class-elsewhere note and the see-also reference note.
552 Petrology
Class structural geology in 551.8
004.7 Peripherals
See also 004.64 for communication devices
Because the DDC is developed on the basis of literary warrant, associative relationships are often treated as equivalence or hierarchical relationships because that is how they are treated in the published literature.
Sometimes, the equivalence relationship is used instead of the associative relationship to incorporate topics in a hierarchy that are related, but not equivalent or subordinate concepts. For example, the equivalence relationship is used in sociology to equate the sociology of religious institutions with the sociology of religion; these are really related concepts.
306.6 Religious institutions
Class here sociology of religion
Polyhierarchical relationships often include associative and hierarchical elements, as seen in the earlier example of meteorite craters. Because craters have been made equivalent to volcanoes at 551.21 through the device of the class-here note, a see reference is used to reflect the resulting structural hierarchical relationship between 551.21 Volcanoes and 551.397 Meteorites for meteorite craters. The relationship between the two categories at the caption level is at best associative.
Conversely, the associative class-elsewhere note is sometimes used to draw attention to subordinate topics in the same hierarchy:
512 Algebra, number theory
Class foundations of algebra in 512.9
3.0 Relationships in the Relative Index
A key feature of the DDC is the Relative Index. The Relative Index is an alphabetical list of subjects with the disciplines in which they are treated subarranged alphabetically under each entry. Because disciplines provide the context for topics in the DDC, the Relative Index provides a useful reverse display of topics and the disciplines in which they appear. The see also reference in the index serves three functions in showing relationships among topics in the index: equivalence (Use), hierarchical (BT), and associative (RT).
The see also reference leads from the nonpreferred form of the index entry to the preferred form (the equivalence function) on the premise that it is useful to give at least one number with the nonpreferred form:
Elderly persons 305.26
T1--0846
see also Older persons
In the example above, Elderly persons is the nonpreferred term; Older persons is the term with full indexing.
The see also reference is used to link index entries to broader concepts under which additional subentries may be found (the hierarchical function):
Player pianos 786.66
see also Mechanical musical instruments
Sometimes, the reference leads to the broader term for a category that may not be the first one considered in a general knowledge organization tool:
Earthquakes 551.22
disaster services 363.3495
social effects 303.485
see also Disasters
Earthquakes are most commonly thought of as part of 551 Geology, but are also a type of disaster.
The see also reference is additionally used to provide links between associated concepts in the Relative Index:
Job hunting 650.14
see also Résumé writing
Résumé writing 808.06665
see also Job hunting
In the schedules and tables, homographs are handled through the hierarchical structure, e.g., "Mercury" appears as the caption at 669.71 (referring to the element) and at 523.41 (referring to the planet). In the Relative Index, these entries are disambiguated by the addition of a qualifier:
Mercury (Element) 669.71
Mercury (Planet) 523.41
If one meaning of the term is the most common one, that meaning is not required to be qualified. For example, cold in the meaning of temperature appears without a qualifier in the Relative Index:
Cold 536.56
Cold (Disease)
medicine 616.205
4.0 Relationships in the Manual
The schedules, tables, and Relative Index all contain references to the Manual. One of the key functions of the Manual is to provide advice on the choice of a number for a topic. This advice often includes discussions on polyhierarchical and associative relationships.
170 vs. 303.372
Ethics (Moral philosophy) vs. Belief systems and customs
Social ethics may be a subject either in moral philosophy or in methods of social control. Social ethics in 170 refers to the rightness or wrongness of conduct as it affects individuals or society. Social ethics in 303.372 refers to beliefs and systems of beliefs influencing the way society and its institutions operate. . .
355 vs. 623
Military science vs. Military and nautical engineering
Use 623 for physical description, design, manufacture, operation, and repair of ordnance; use 355-359 for procurement and deployment, and also for the units and services that use the ordnance. . .
5.0 Relationships in Synthesized Notation
Only a fraction of potential DDC numbers are explicitly listed in the DDC. It is often necessary to synthesize (build) a number in order to represent a concept. Each synthesized number brings together elements from multiple hierarchies in a syntactic relationship. There are four sources of notation from which to build numbers: (A) Table 1 Standard Subdivisions; (B) Tables 2-7; (C) other parts of the schedules; and (D) add tables in the schedules. Standard subdivisions may be added for any concept that is equivalent to the category. Other numbers may be synthesized upon instruction.
Table 1 is used to add aspects such as form and time, and to introduce aspects from other tables such as place (Table 2) or ethnic group (Table 5):
A medical journal 610.5
61 Medicine (from 610) [topic]
05 Journals (Table 1) [form]
Computer science in the 1970s 004.09047
004 Computer science [topic]
09047 1970-1979 (Table 1) [time]
The Internet in Scandinavian countries 004.6780948
004.678 Internet [topic]
09 Geographic treatment (Table 1)
48 Scandinavia (Table 2) [place]
Jewish ceramic arts 738.089924
738 Ceramic arts [topic]
089 Racial, ethnic, national groups (Table 1)
924 Jews (Table 5) [group]
In the examples above, the Table 1 notation serves as a type of relational operator. In many places in the DDC, the operator, or facet indicator, is eliminated to shorten the notation, and the aspect is added directly to the base number through an instruction in the schedule:
Higher education in France 378.44
378 Higher education [topic]
44 France (Table 2) [place]
As already illustrated, Table 2 provides geographic notation. Table 3 is used to introduce forms and genres in literature and (to a limited extent) in the arts. Table 4 is used to extend aspects of languages. Table 5 provides notation for racial, ethnic, and national groups. Table 6 provides notation for specific languages. Table 7 provides notation for groups of persons. (Note: Table 7 will be eliminated in the next edition of the DDC because notation for the same concepts is already available in Table 1 or in the schedules.)
Numbers are also synthesized from internal tables and instructions for direct addition from the schedules:
Vertebrate mechanics 571.316
571.3 Morphology
1 Facet indicator for animals (from 571.1 Animals)
6 Vertebrates, from 596
In this example, two hierarchies are linked together: the hierarchy for morphology/physiology/biology (570) and the hierarchy for vertebrates/animals (590).
Another example is the nocturnal behavior of bats, in which two hierarchies in the same division of the DDC (590) are linked together:
Nocturnal behavior of bats 599.41518
599.4 Bats
1 Facet indicator for general topics (from table under 592-599)
518 Nocturnal behavior, from 591.518
The bats example also can be used to illustrate another useful feature of the DDC: meaningful notation. Across the DDC, the notation 94 (derived from 599.94) is used to represent "bats" the mammal; other notation is used to represent "bats" the sports equipment.
Bats 599.4
conservation technology 639.9794
physiology 571.194
Bats (Sports equipment)
baseball 796.35726
manufacturing technology 688.76357
cricket 796.35826
manufacturing technology 688.76358
Bats
6.0 Intradisciplinary and Interdisciplinary Relationships
In our earlier example of craters, we illustrated how intradisciplinary relationships among aspects of a topic are handled. What about topics that cross disciplines? Relationships among disciplines are often addressed directly in the schedules:
305.231 Child development
Class here interdisciplinary works on child development
For psychological development of children, see 155.4; for physical development of children, see 612.65
The same set of relationships is displayed in the Relative Index; the interdisciplinary number appears opposite the main index entry:
Child development 305.231
physiology 612.65
psychology 155.4
sociology 305.231
The Manual also contains advice on the relationships among disciplines for certain topics. For example, the following discussion of cognitive science, a topic important in psychology and artificial intelligence, appears at 153 vs. 006.3 in the Manual:
Class cognitive science . . . in 153 if the goal is to understand better how the human mind works. Class cognitive science in 006.3 if the goal is to produce computer systems with better artificial intelligence. If in doubt, prefer 153.
Of course, the final advice could be ignored in nonlinear application of the DDC, i.e., both numbers could be assigned.
7.0 Flexible Structures
One method of extending the relationships in the DDC is through the introduction of flexible structures. A flexible structure is an alternative view that is derived from or linked to a general organization scheme to address an information need that is not easily accommodated through the existing structure (Mitchell, 1998).
7.1 Derived Relationships
The abridged edition of the DDC is the simplest flexible structure that may be derived from the DDC. It is a logical abridgment of the full edition of the DDC based on the literary warrant of small library collections. Abridged numbers provide an alternative for smaller collections to use logically broader notation in a less complex structure. One unfortunate side effect of departures from notational hierarchy in the full edition is that the same structure must be accommodated in the abridged edition. For example, since Norway and its parts are on coordinate notation in the full edition, the abridged edition must also include the coordinate notation for Norway and its parts in order to avoid false abridgment.
Flexible structures may also be introduced through options and translations. Throughout the DDC, alternative relationships are provided through options. Options provide alternatives to the standard structure in terms of jurisdictional emphasis; racial, ethnic, national group emphasis; language emphasis; topical emphasis; or emphasis by some other special characteristic (Mitchell, 1995). A simple option is to give prominence to a literature other than American literature in English by moving both literatures to different positions in the hierarchical structure. One option is to vacate 810 and merge American literature in English with 820 English literature, thus freeing 810 for a literature of local importance regularly at a deeper level in the hierarchy. For example, the Vietnamese translation of the DDC could merge the standard meaning of 810 with 820, and use 810 for Vietnamese literature (895.922 in the English-language standard edition).
Translations of the latest editions of the DDC are currently underway in Arabic, Chinese, Greek, Hebrew, Icelandic, Italian, Korean, Norwegian, Russian, and Spanish. The French translation of Edition 21 was published in 1998 (Dewey, 1998). German and Vietnamese translations are under discussion. In a translation, expansions or use of options may alter some of the existing relationships in the English-language standard edition. For example, in Table 2 in the Italian translation (Dewey, 1993), the equivalence relationship between the province and city of Bologna has been replaced by a hierarchical relationship through expansion:
--4541 Provincia di Bologna
--45411 Comune di Bologna
Several researchers are exploring derived views of the DDC. Cochrane and Johnson (1996) have proposed use of the thesaural relationships in Dewey as an aid to bibliographic searching. Pollitt (1998) is testing the derivation and presentation of facets from Dewey as an aid to online searching. Vizine-Goetz (forthcoming) is experimenting with the development of pathfinders accessible from a restructured set of DDC classes.
7.2 Mapped Relationships
Relationships within the DDC can be dynamically altered through the linking of thesauri and subject heading lists to the system. The linking of other thesauri enriches the vocabulary in the DDC, and provides an alternative view within the general organizational framework of the DDC. A mapped thesaurus enables the DDC to offer domain-specific vocabulary and relationships to the specialist. The specialist can simultaneously use the DDC's general knowledge framework for the exploration of interdisciplinary or tangential topics. Olson and Ward (1998) are experimenting with mapping A Women's Thesaurus to the DDC to overlay the relationships found in women's studies on the existing Dewey structure. Iyer and Giguere (1995) have proposed seven relationships for the linking of the American Mathematics Society Mathematics Subject Classification to the DDC:
(1) Exact matches
(2) Specific to general
(3) General to specific
(4) Many to one
(5) Cyclic mapping strategies
(6) No matches
(7) Specific and broad class mapping
Vizine-Goetz (1999) has developed various mapping strategies for Library of Congress subject headings, along with assessments of the strength of the association. The strength of the association varies depending upon the method used to link class numbers with headings.
CORC Abbreviation |
Source of Term |
Association Method |
Strength of Relationship |
IM |
Editorially mapped LCSH |
Intellectual |
Strong |
SHC |
Subject Headings for Children |
Intellectual |
Strong |
NF |
LCSH assigned in NetFirst records |
Intellectual |
Moderate to Strong |
SM |
OCLC WorldCat |
Statistical |
Moderate |
FM |
Dewey for Windows (from WorldCat) |
Statistical |
Weak to Moderate |
Terms with the IM designation have the strongest association--Dewey editorial staff members have intellectually linked them to the DDC. Terms with an SHC designation are also strongly associated with the DDC, but often at a broader number that represents the concept in the abridged edition of the DDC. These LC subject headings were extracted from the WorldCat database by processing LC MARC records with headings used in the LC Annotated Card Program (AC)/Subject Headings for Children's Literature, then the candidate mappings were reviewed by the Dewey editors to match numbers in Abridged Edition 13. OCLC NetFirst editors have linked the LC subject headings with the NF designation to DDC numbers in records for Internet-accessible resources. The two remaining sets of terms have been automatically linked to the DDC using statistical techniques. The abbreviation SM is used to identify terms associated with the DDC using a term co-occurrence measure. This measure was applied to over 710,000 WorldCat records that contain MARC 082 (Dewey Decimal Classification number) and a subject heading field, MARC tag 600-651. The terms labeled FM consist of LC subject headings that are included in the Dewey for Windows database through straight frequency mapping from WorldCat.
There is still much work to be done on coding the nature of mapped relationships, and how the purpose for which the relationship is to be used will affect the definition of the link (Mitchell, 1998).
8.0 Assessment of Relationships in Dewey
The nature of relationships in Dewey is richer than is apparent from a linear view of the system. As discussed in this chapter, the hierarchical relationship goes beyond that of the notation itself and is embedded in the structure of headings, notes, and entries in the DDC. No special labeling exists for the different kinds of hierarchical relationships, but the type can often be determined from the context in which it appears (e.g., a taxonomic schedule, parts of a discipline, a listing of geographic features). The DDC stretches the equivalence relationship to accommodate literary warrant and practical classification concerns. The associative relationship is employed liberally throughout the DDC to connect related topics distributed within and among disciplines.
Each synthesized number brings together elements from multiple hierarchies in a syntactic relationship. Additional aspects are often introduced by special notation, e.g., the addition of geographic area through 09. In some instances, the facet indicator is eliminated to shorten the notation, thus resulting in a loss of explicit relational information.
Direct revision is not the only answer to improving relationships in the DDC. Flexible structures that are derived from or linked to the DDC show great promise in modifying and expanding the network of relationships in the system.
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