Classification Systems

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Classiﬁca on Systems

Alberto Simões
ambs@ilch.uminho.pt

March 14th , 2012

Alberto Simões Classiﬁca on Systems

. Classifica on Systems

Humans tend to organize;
``disorganiza on is a kind of organiza on''
This organiza on is usually done by classifica on;
Classifica on can be as simple as tagging an object;
``this is the pile of important documents, that of the
non-important ones''
Classifica on is used everywhere!


. Where are classifica on systems used?

Internet Social Networks (tagging);
Libraries (universal decimal classifica on);
Medicine (illness classifica on);
Chemistry (periodic table);
Geography (geographic taxonomy);
Biology (protein classifica on);
Linguis cs (languages classifica on);


. Classifica on Systems Classes

Classifica on Systems can also be classified;
One way to classify classifica on systems is by their ability to
include proper es and rela ons between the classified objects;
We will discuss four types of classifica on systems:
Folksonomies
Taxonomies
Thesauri
Ontologies


. Class Task

A B C

D E F

. Folksonomies


. Folksonomies

A folksonomy is a system of classiﬁca on derived from
the prac ce and method of collabora vely crea ng and
managing tags to annotate and categorize content; this
prac ce is also known as collabora ve tagging, social
classiﬁca on, social indexing, and social tagging. Folk-
sonomy, a term coined by Thomas Vander Wal, is a port-
manteau of folk and taxonomy.

Folksonomy (Wikipedia, 2012)


. Folsksonomies: How they work

The other classifica on techniques we will see, define someone
or some group in charge of crea ng the classifica on system
structure (authority);
This group of people see the world from a specific point of view,
that can be, or not, shared by others;
Folksonomies solve this problem: power to the people;
Instead of par oning the world accordingly with a view, lets
the user present facets of objects;
Users assign keywords (or tags, or labels) to objects (individuals);
These keywords can be searched, indexed, and mathema cal
models can be applied to this data.


. Folksonomies
An empirical analysis of the complex dynamics of tag-
ging systems, published in 2007, has shown that consen-
sus around stable distribu ons and shared vocabularies
does emerge, even in the absence of a central controlled
vocabulary. For content to be searchable, it should be
categorized and grouped. While this was believed to
require commonly agreed on sets of content describing
tags (much like keywords of a journal ar cle), recent re-
search has found that, in large folksonomies, common
structures also emerge on the level of categoriza ons.
Accordingly, it is possible to devise mathema cal mod-
els that allow for transla ng from personal tag vocab-
ularies (personomies) to the vocabulary shared by most
users.

Folksonomy (Wikipedia, 2012)

. Folksonomies: example
Top categories in the PT Wikipedia (those that don't have spaces):
375 Sociologia 383 Ponerinae
395 Afro-brasileiros 404 Drilliidae
413 Filosofia 415 Coleophoridae
424 Psicologia 428 Terebridae
445 Clathurellinae 445 Digimons
445 Teuto-brasileiros 451 Apiaceae
483 Asteroides 486 Luso-brasileiros
492 Acaena 526 Rubiaceae
537 Dolichoderinae 730 Agonoxenidae
735 Acalypha 753 Mangeliinae
762 Crambidae 787 Poaceae
808 Coletâneas 824 Theraphosidae
854 Myrmicinae 962 Fabaceae
974 Formicidae 1065 Agrostis
1096 Formicinae 1177 Aloe
1328 Conus 1338 Ítalo-brasileiros
1395 Asteraceae 1433 Coleophora
1514 Arctiidae 1516 Alchemilla
1689 Turridae 1879 Camponotus
2163 Acer 2744 Acacia


. Folksonomies: Pros and Cons

Pros:
doesn't require expert cataloguers, authorita ve sources or
expert users;
capability of matching users' real needs and language:
(inclusive --- includes everyone's words and vocabulary)
controlled vocabularies are not prac cally and economically
extensible;
a low-investment bridge between personal classiﬁca on and
shared classiﬁca on;
easy to use and quick to classify big quan es of individuals;
not all the limita ons of folksonomies are defects :-)


. Folksonomies: Pros and Cons

Cons:
by itself, the vocabulary is flat;
(there is no structure, just terms)
not usable for small or few users collec ons;
(sta s c methods significance is dependent on popula on size)
without some technology help, vocabularies get inexact or
ambiguous;
have a very low findability quo ent. They are great for
serendipity and browsing but not aimed at a targeted approach
or search;


. Taxonomies


. Taxonomies

Taxonomy is the science of iden fying and naming
species, and arranging them into a classifica on. The
field of taxonomy, some mes referred to as ``biological
taxonomy'', revolves around the descrip on and use of
taxonomic units, known as taxa. A resul ng taxonomy
is a par cular classifica on, arranged in a hierarchical
structure or classifica on scheme.

Taxonomy (Wikipedia, 2012)


. Taxonomies

taxonomy [tækˈsɒnəmɪ]
n.
(Life Sciences & Allied Applica ons / Biology)
the branch of biology concerned with the
classiﬁca on of organisms into groups based on
similari es of structure, origin, etc.
the prac ce of arranging organisms in this way.
the science or prac ce of classiﬁca on.
[from French taxonomie, from Greek taxis order + -
nomy]

Collins English Dic onary – Complete and Unabridged
© HarperCollins Publishers 1991, 1994, 1998, 2000, 2003


. Taxonomies: How they work?

Used to par oning the world in disjunc ve classes or groups;
Each group is, again, par oned in sub-classes or sub-groups;
And sub-classes are par oned, and…

Individuals are classiﬁed in one leaf category;
(a classiﬁca on is a path in the tree)


. Taxonomies: The typical example


. Taxonomies: The example you use everyday
Main index (top level) of Universal Decimal Classiﬁca on:
0 Generali es
(now Science and knowledge. Organiza on. Computer Science.
Informa on. Documenta on. Librarianship. Ins tu ons. Publica ons)
1 Philosophy. Psychology
2 Religion. Theology
3 Social Sciences
4 Vacant
5 Mathema cs and natural sciences
6 Applied sciences. Medicine. Technology
7 The arts. Recrea on. Entertainment. Sport
8 Language. Linguis cs. Literature
9 Geography. Biography. History

. Taxonomies: The example you use everyday
80 General ques ons […] linguis cs and literature. Philology
81 Linguis cs and languages
81-11 Schools and trends in linguis cs
81-13 Methodology of linguis cs. Methods and means
811 Languages
811.1/.2 Indo-European Languages
811.3 Dead languages of unknown affilia on. Caucasian languages
811.4 Afro-Asia c, Nilo-Saharan, Congo-Kordofanian, Khoisan languages
811.5 Ural-Altaic, Palaeo-Siberian, Eskimo-Aleut, Dravidian and
Sino-Tibetan languages. Japanese. Korean…
811.6 Austro-Asia c languages. Austronesian languages
811.7 Indo-Pacific (non-Austronesian) languages. Australian languages
811.8 American indigenous languages
811.9 Ar ficial languages

82 Literature


. Taxonomies: Class Task
0 Science and knowledge. Organiza on.
Computer Science. Informa on…
1 Philosophy. Psychology
2 Religion. Theology
3 Social Sciences
5 Mathema cs and natural sciences
6 Applied sciences. Medicine. Technology
7 The arts. Recrea on. Entertainment.
Sport
9 Geography. Biography. History Prolog Programming for
Ar ﬁcial Intelligence,
Prof Ivan Bratko

5 Mathema cs, Natural Sciences
51 Mathema cs
519 (no name, virtual class)
519.6 Computa onal mathema cs.
Numerical Analysis

Prolog Programming for
Prof Ivan Bratko
SDUM

00 Prolegomena. Fundamentals of
knowledge and culture.
Propaedeu cs
004 Computer science and technology.
Compu ng. Data processing
004.4 So ware
004.42 Computer programming.
Computer programs

UA Ar ﬁcial Intelligence,
Prof Ivan Bratko

Propaedeu cs
004.4 So ware
004.43 Computer Languages

IPP Prof Ivan Bratko

Propaedeu cs
004.8 Ar ﬁcial intelligence

UAlg
Prof Ivan Bratko

. Taxonomies: Pros and Cons
Pros:
rigid tree, makes it easy to process;
suitable for some areas (like life classifica on);
the hierarchy helps searching for terms (abstrac on);
Cons:
rigid tree, makes it difficult to classify;
(different people classify objects differently)
the structure is defined by some authority group;
(for example, the UDC Consor um)
forces the subdivision of the world;
(categories are single-parental)
as a workaround, people classify in more than one category;
(so, the rigid tree Pro gets a Con)

. Thesauri


. Thesauri

A thesaurus is a reference work that lists words grouped
together according to similarity of meaning (contain-
ing synonyms and some mes antonyms), in contrast to
a dic onary, which contains deﬁni ons and pronuncia-
ons.

Thesauri (Wikipedia, 2012)


. Thesauri

In Informa on Science, Library Science, and Informa on
Technology, specialized thesauri are designed for infor-
ma on retrieval. They are a type of controlled vocabu-
lary, for indexing or tagging purposes. Such a thesaurus
can be used as the basis of an index for online material.
[…] Unlike a literary thesaurus, these specialized thesauri
typically focus on one discipline, subject or ﬁeld of study.



. Thesauri: How they work!
Thesauri for informa on retrieval are typically con-
structed by informa on specialists, and have their own
unique vocabulary deﬁning diﬀerent kinds of terms and
rela onships.

Terms are the basic seman c units for conveying con-
cepts. They are usually single-word nouns, since nouns
are the most concrete part of speech. […] When a term
is ambiguous, a ``scope note'' can be added to ensure
consistency, and give direc on on how to interpret the
term.

``Term rela onships'' are links between terms. These re-
la onships can be divided into three types: hierarchical,
equivalency or associa ve.



Hierarchical rela onships are used to indicate terms
which are narrower and broader in scope. A ``Broader
Term'' (BT) or hyperonym is a more general term. Recip-
rocally, a ``Narrower Term'' (NT) or hyponym is a more
speciﬁc term.
BT and NT are reciprocals; a broader term necessarily
implies at least one other term which is narrower. BT
and NT are used to indicate class rela onships, as well
as part-whole rela onships (meronyms and holonyms).



Example of a thesaurus with hierarchical rela ons.

Feline
NT Cat
NT Panther

Cat
BT Feline

Panther
BT Feline
NT Pink Panther

Pink Panther
BT Panther



The equivalency rela onship is used primarily to con-
nect synonyms and near-synonyms. `Ùse'' (USE) and
`Ùsed For'' (UF) indicators are used when an autho-
rized term is to be used for another, unauthorized,
term. Unauthorized terms are o en called `èntry vo-
cabulary'', `èntry points'', ``lead-in terms'', or ``non-
preferred terms'', poin ng to the authorized term (also
referred to as the ``preferred term'' or ``descriptor'') that
has been chosen to stand for the concept.




Example of a thesaurus with equivalency rela ons.

Parliament
USE European Parliament

Parliament of Europe
USE European Parliament

European Parliament
UF Parliament
UF Parliament of Europe



Associa ve rela onships are used to connect two re-
lated terms whose rela onship is neither hierarchical
nor equivalent. This rela onship is described by the in-
dicator ``Related Term'' (RT). Associa ve rela onships
should be applied with cau on, since excessive use of RTs
will reduce speciﬁcity in searches.
Consider the following: if the typical user is searching
with term "A", would they also want resources tagged
with term "B"? If the answer is no, then an associa ve
rela onship should not be established.




Example of a thesaurus with associa ve rela ons.

Douro Porto
BT River BT Portugal
RT Porto
RT Gaia Portugal
NT Porto
River NT Gaia
NT Douro
City
Gaia NT Gaia
BT Portugal NT Porto

Note RT is not symmetrical. a RT b ̸⇒ b RT a.


. Thesauri: a simple example

Extract of Food Safety relationships in AGROVOC

Quality Asia

BT BT

NT NT

RT RT
Food Safety Contamination China

USE

USE
Food Food Contamination


. Thesauri: Pros and Cons

Pros:
More ﬂexible than Taxonomies;
(does not require a tree, work as a graph)
Have other types of rela onship than simple hierarchy;
There is an ISO standard that documents their correct use;
Standard deﬁnes mathema cal proper es for rela onships;
Cons:
Standardized types of rela onship are kind of limited;
(same rela on for hyperonyms and meronyms)
(non-hierarchical rela on is too vague: related)
No support for rela onships with non-terms (features);


. Ontologies


. Ontologies

Ontology is the philosophical study of the nature of be-
ing, existence, or reality as such, as well as the basic cat-
egories of being and their rela ons. Tradi onally listed
as a part of the major branch of philosophy known as
metaphysics, ontology deals with ques ons concerning
what en es exist or can be said to exist, and how such
en es can be grouped, related within a hierarchy, and
subdivided according to similari es and diﬀerences.

Ontology (Wikipedia, 2012)


. Ontologies

In computer science and informa on science, an ontol-
ogy formally represents knowledge as a set of concepts
within a domain, and the rela onships between those
concepts. It can be used to reason about the en es
within that domain and may be used to describe the do-
main.

Ontology: informa on science (Wikipedia, 2012)


. Ontologies

Contemporary ontologies share many structural simi-
lari es, regardless of the language in which they are
expressed. Most ontologies describe individuals (in-
stances), classes (concepts), a ributes, and rela ons.



. Ontologies

Individuals are the instances or objects (the basic or
``ground level'' objects).


Unlike any of the other classiﬁca on systems, Ontologies clearly
include the individuals (or objects being classiﬁed) in the structure.


. Ontologies

Classes are sets, collec ons, concepts, […] or kinds of
things.


Classes are the concepts used in Thesauri and Taxonomy. They can be
super-classes, including sub-classes, or can just include individuals
(low level classes, leafs if we were talking about taxonomies).


. Ontologies

A ributes are aspects, proper es, features, characteris-
cs, or parameters that objects (and classes) can have.


A ributes are proper es of individuals or classes. If the individual is a
book on a library, a property can be the number of pages, the tle,
the author. For a class, like ``mammal'', an a ribute can be a
reference to its fur.


. Ontologies

Rela ons are ways in which classes and individuals can
be related to one another.


Rela ons are similar to the rela ons used in Thesauri, but unlike
them, there isn't a list of valid rela ons. They can be the common
hierarchical rela ons, or the rela on ``eat'' rela ng animals with the
animals they eat.


. Ontologies

Func on terms: complex structures formed from certain
rela ons that can be used in place of an individual term
in a statement.


Suppose you are adding Portuguese rivers to an Ontology. One can
deﬁne a simple macro to add some default rela ons to the river:

 Term → name
∼
River(name)= Is a → river

Is at → Portugal


. Ontologies

Restric ons: formally stated descrip ons of what must
be true in order for some asser on to be accepted as in-
put.


We can force that a capital of a country it a city:

add (X capital-of Y) iﬀ X is-a City


. Ontologies

Rules: statements in the form of an antecedent-
consequent sentence that describe the logical inferences
that can be drawn from an asser on in a par cular form.


In the other hand, if we trust who is edi ng an ontology, we can
classify automa cally it as a city, and its country as a…country:

X capital-of Y⇒X is-a City ∧ Y is-a Country


. Ontologies

Axioms: asser ons (including rules) in a logical form that
together comprise the overall theory that the ontology
describes in its domain of applica on.


Diﬀers from Rules in the aspect as axioms are tests to guarantee the
ontology structure. They are not used to infer new rela ons. They
assert.


. Ontologies

Events: the changing of a ributes or rela ons.


Similar to rules, but react to events. For example, if the user adds a
feature sta ng that an individual lays eggs, classify it as an oviparous.
Note that the division in Rules, Axioms and Events is not universal,
and depend quite a lot in the applica on that is used to support the
ontology.


. Ontologies: Example 1


. Ontologies: Example 2


. Ontologies: Pros and Cons

Pros:
More flexible than Thesauri;
(graph with ad-hoc rela onships)
Lots of formalisms and standards (OWL, SKOS, …);
Lots of tools to edit (like Protégé);
Languages for querying and comple on (like SPARQL);
Cons:
As a classifica on approach, requires an authority for its
defini on, just like Taxonomies or Thesauri.
Complexity: not everybody is able to create a detailed ontology.


. Further Reading
Folksonomies:
Folksonomy Coinage and Deﬁni on
http://vanderwal.net/folksonomy.html
Folksonomies: A User-Driven Approach to Organizing Content
http://www.uie.com/articles/folksonomies/
Folksonomies: power to the people
http://www.iskoi.org/doc/folksonomies.htm
Folksonomies: Tidying up Tags?
http://www.dlib.org/dlib/january06/guy/01guy.html
Folksonomies - Coopera ve Classiﬁca on and Communica on
Through Shared Metadata
http://www.adammathes.com/academic/
computer-mediated-communication/folksonomies.html


. Further Reading
Taxonomies:
Taxonomy
http://en.wikipedia.org/wiki/Taxonomy
Perspec ves on Taxonomy, Classiﬁca on, Structure and
Find-ability
http://www.serviceinnovation.org/included/docs/
kcs_taxonomy.pdf
Universal Decimal Classiﬁca on
http://www.udcc.org/udcsummary/php/index.php

Thesauri:
Thesaurus
http://en.wikipedia.org/wiki/Thesaurus
Thesaurus principles and prac ce
http://www.willpowerinfo.co.uk/thesprin.htm

. Further Reading

Ontologies:
Ontology (informa on science)
http://en.wikipedia.org/wiki/Ontology_
(information_science)
Protégé Ontology Editor
http://protege.stanford.edu/
OWL Web Ontology Language
http://www.w3.org/TR/owl-features/
SPARQL Query Language for RDF
http://www.w3.org/TR/rdf-sparql-query/


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