Crosslingual Ontology-Based

Document Retrieval (Search)

in an eLearning Environment

Eelco MosselLSP 2007, Hamburg

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• EU-Project LT4eL: Language Technology for eLearning (www.lt4el.eu)

• Goal: use of Language Technology to improve the effectiveness of Learning Management Systems

• Multilingual Setting: 8 languages• 12 European partner universities/institutes• Crosslingual search: work together with:

– Cristina Vertan (University of Hamburg)– Kiril Simov (Bulgarian Academy of Sciences, Sofia)– Alex Killing (ETH Zürich (Eidgenössische Technische

Hochschule))

Framework

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• Project framework• Learning Management System• Goals of semantic search• Resources for search function• Features (user side)• Internal components (developer side)• Evaluation

Overview

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• Learning Platform on the internet (interactive website)– Users can log in, have a profile, chat, …

• Fundamental: store and access Learning Material• Units: Learning Objects (LOs) = documents• Test-System : open source platform ILIAS (www.ilias.de/)

• Test material: text-based documents in 8 languages (PDF, HTML, MS-Word)

• Domain of test material: Computer Science for non CS specialists

Learning Management System (LMS)

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Goals of the approach1. Improved retrieval of documents

– Find documents that would not be found by simple text search (exact search word occurs in text)

– Example: search for “screen” – retrieve doc that contains “monitor” but not “screen”.

2. Multilinguality– One implementation for all languages in the project

3. Crosslinguality– Find documents in languages different from

search/interface language• No need to translate search query• Search possible with passive foreign language knowledge

Crosslingual semantic search

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• Approach: use a domain ontology• Creation:

– Select keywords from LOs (also used for another project goal/task)

– Choose keywords relevant for the domain “Computer Science for Non-Computer Scientists)

– Derive a set of concepts from the set of keywords. Concepts have English names/labels

– Provide a definition in English for each concept– Create OWL taxonomy/ontology from concepts,

by specifying relations between domain concepts, and mapping to DOLCE and WORDNET ontology.

– 1 ontology, for language-independent use, but contains English as common language for labels and definitions

– Currently 707 concepts

Resource: domain ontology

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• Connection between terms (words in a certain language) and concepts

• Create term-concept lexicons:– For each language and each concept,

specify terms (synonyms if relevant) that denote the concept

– At least one term for each concept– German: currently 939 terms 707

concepts

Resource: term-concept lexicons

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• Concept Document relations• Annotate concepts in documents in a

semi-automatic way using the lexicons:– Occurrences of terms are annotated

with corresponding concepts– Annotator (person) decides whether or

not to annotate this occurrence, and chooses between concepts for ambiguous terms

Resource: concept-annotations

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Starting points

• A multilingual document collection• An ontology including a domain ontology on

the domain of the documents• Concept lexicalisations in different

languages• Annotation of concepts in the documents

Crosslingual semantic search

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CommunicationsProtocol

HTTP FTP

Docs L1

Docs L3

Docs L2

HTTP

SSL

FTPIP

Terms L1

Terms L3

Terms L2

Lexicons in different languages

search terms

browse and select

Connecting the components

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Search with ILIAS

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• Search using search words, concepts from the ontology or a combination of both– In case of combination: results of directly selected

concepts come first• Search for documents with super- or subconcepts

– For documents in which desired concept is not found• Ranking

– Number of different searched concepts in document– Normalised annotation frequency– Super/subconcepts have lower weight

• Shared concepts (occurring in e.g. 50% of the found documents)– Example:

Concept “Report” Some documents about academic writing Concept “Publication”

• Navigate through ontology (get related concepts)

Features of the search functionality

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Search functionality comprises:

1. Find terms in lexicons that reflect search query.

2. Find corresponding concepts for derived terms.

3. Find relevant documents for concepts. 4. Create ranking for set of found

documents.5. Create ontology fragment containing

necessary information to present concept neighbourhood

6. Find “shared concepts”

Internal components

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• Why start with a free text query?– User wants results fast (as in Google)– Compete with fulltext search and keyword search– Find starting point for ontology browsing

• Query lexicon: adopted/implemented strategies for– Tokenise create combinations for multiword terms (e.g.

"space bar"), – Loose match of diacritic and uppercase letters (é e; E

e)• Other ideas to improve recognition of query:

– Lemmatisation of search terms– Expansion of lexicon with word forms– Match similar strings

• Insertion of function words e.g. “provedor acesso” “provedor de acesso”

– Automatic substring match - Dynamic list of available terms that contain input so far

1: Query Terms

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Not always 1:1 mapping.• Corresponding concept is missing from ontology

– LT4eL: not in lexicon• Unique result: term is lexicalisation of one concept• Multiple concepts from one domain, e.g.:

– Key (from keyboard)– Key (in database)

• Concepts from more domains: – Window (graphical representation on monitor)– Window (part of a building)

• Different concepts for different languages:– “Kind” (English: sort/type)– “Kind” (German: child)

Let the user choose: present multiple browsing units

2: Term Concept

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• Simplest: – Disjunctive search with ranking

• For each concept, each document that is annotated with it is returned

• Documents with more desired concepts are ranked higher

• Use super/subconcepts• Further possibilities

– Conjunctive search:• Combination of concepts must occur in a document• Is taken into account in current ranking

– Context search:• Combination of concepts must occur in a paragraph or

sentence– Word & Concept search combined:

• Document must contain concepts as well as certain words

3: Concept Documents

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• How useful is it, to find documents that treat a superconcept?– Negative example: lt4el:Subroutine lt4el:Software.

Other children of Software are e.g.: Shareware, AuthoringLanguage

– Positive example: GraphicalUserInterface UserInterface

• How useful is it, to find documents that treat a subconcept?– lt4el:Program has 93 subconcepts, e.g.:

• ApplicationProgram• Computervirus• Driver• Unzip

3: Concept Documents (continued)

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• Does semantic search return correct results? (appropriate documents)

• How easy is it to use semantic search?• Are the results better (precision/recall)

than with keyword search or fulltext search (also available in ILIAS)?– Relevant for monolingual scenario

• Is the learning process improved?– Depends on quality of ontology and annotation– In multilingual case: depends on domain

knowledge and language knowledge of multilingual test persons

Evaluation

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Thank you