OntoQA: Metric-Based Ontology Quality Analysis

Samir Tartir, I. Budak Arpinar, Michael Moore, Amit P. Sheth,

Boanerges Aleman-Meza

IEEE Workshop on Knowledge Acquisition from Distributed, Autonomous, Semantically Heterogeneous Data and Knowledge

Sources

Houston, Texas, November 27, 2005

The Semantic Web• Current web is intended for human use• Semantic web is for humans and

computers• Semantic web uses ontologies as a

knowledge-sharing vehicle.• Many ontologies currently exist: GO, OBO,

SWETO, TAP, GlycO, PropreO, etc.

Motivation

• Having several ontologies to choose from, users often face the problem of selecting the best ontology that is suitable for their needs.

OntoQA• Metric-Based Ontology Quality Analysis

• Describes ontology schemas and instancebases (IBs) through different sets of metrics

• OntoQA is implemented as a part of SemDis project.

Documentsdatabases

Open/proprietary Heterogeneous Data Sources

HtmlXMLfeeds

PopulatedOntology

Ontology Schema

Emails

Contributions

• Defining the quality of ontologies in terms of:• Schema• Instances

• IB Metrics• Class-extent metrics

• Providing metrics to quantitatively describe each group

I. Schema Metrics• Schema metrics address the design of the

ontology schema.

• Schema quality could be hard to measure: domain expert consensus, subjectivity etc.

• Three metrics:– Relationship richness– Attribute richness– Inheritance richness

I.1 Relationship Richness

• How close or far is the schema structure to a taxonomy?

• Diversity of relations is a good indication of schema richness.

PIsA

PRR

|P|: Number of non-IsA relationships

|IsA|: Number of IsA relationships

I.2 Attribute Richness

• How much information do classes contain?

C

AAR

|A|: Number of literal attributes

|C|: Number of classes

I.3 Inheritance Richness (Fan-out)• General (e.g. spanning various domains) vs.

specific

C

C,CHCC

ijC

SiIR

|Hc(cj, ci)|: Number of subclasses of Class Ci

|C|: Number of classes

II. Instance Metrics• Deal with the size and distribution of the

instance data.

• Instance metrics are grouped into two subcategories:

1. IB metrics: describe the IB as a whole2. Class metrics: describe the way each class that is

defined in the schema is being utilized in the IB

II.1.a Class Richness

• How much does the IB utilizes classes defined in the schema?

• How many classes (in the schema) are actually populated?

C

CCR

`

|C’|: Number of used classes

|C|: Number of defined classes

II.1.b Average Population

• How well is the IB “filled”?

C

IP

|I|: Number of instances

|C|: Number of defined classes

II.1.c Cohesion

• Is IB graph connected or disconnected?

CCCoh

|CC|: Number of connected components

II.2.a Importance

• How much focus was paid to each class during instance population?

I

)I(CImp i

Ci

|Ci(I)|: Number of instances defined for class Ci

|I|: Number of instances

II.2.b Connectivity

• What classes are central and what are on the boundary?

C}C(I),CI(I)CI)I,P(I :{IConn jjjiijijCi

P(Ii,Ij): Relationships between instances Ii and Ij.

Ci(I): Instances of class Ci.

C: Defined classes.

II.2.c Fullness

• Is the number of instances close to the expected?

|)I`(C|

)I(CF

i

i

|Ci(I)|: Number of instances of class Ci.

|Ci’(I)|: Number of expected instances of class Ci.

II.2.d Relationship Richness

• How well does the IB utilize relationships defined in the schema?

)C,C(P

}CC),I(CI),I(CI:))I,I(P(Distinct{RR

ji

jjjiiji

Ci

P(Ii,Ij): Relationships between instances Ii and Ij.

Ci(I): Instances of class Ci.

Cj(I): Instances of class Cj.

C: Defined classes

P(Ci,Cj): Relationships between instances Ci and Cj.

II.2.e Inheritance Richness

• Is the class general or specific?

'C

C,CH

IR'CC

jkC

Cj

i

C’: Classes belonging to the subtree rooted at Ci

|Hc(ck, cj)|: Number of subclasses of Class Ci

Implementation

• Written in Java

• Processes ontology schema and IB files written in OWL, RDF, or RDFS.

• Uses the Sesame to process the ontology schema and IB files.

Testing• SWETO: LSDIS’ general-purpose ontology that covers

domains including publications, affiliations, geography and terrorism.

• TAP: Stanford’s general-purpose ontology. It is divided into 43 domains. Some of these domains are publications, sports and geography.

• GlycO: LSDIS’ ontology for the Glycan Expression

• OBO: Open Biomedical Ontologies

Results – Class Metrics

Ontology # of Classes

# of Instances

Inheritance Richness

Class Richness

Average Population

SWETO 44 1,003,021 0.9 56.8% 22,795.9

TAP 3,230 71,487 1.2 9.4% 22.1

GlycO 356 387 1.3 18.0% 1.1

PropreO 244 0 1.0 0.0% 0.0

Results – Class Importance

Class Importance

010203040506070

Public

atio

n

Scie

ntif

ic_P

ublic

atio

n

Com

pute

r_S

cie

nce_

Researc

her

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aniz

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n

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pany

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rence

Pla

ce

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orist_

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Event

AC

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ubje

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ripto

rs

Class

Class Importance

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101520253035

Mus

icia

n

Ath

lete

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hor

Act

or

Mov

ie

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sona

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pute

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ame

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k

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tate

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tune

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pan

y Ast

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trip

Class

SWETO TAP

GlycO

Class Importance

010203040506070

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Results – Class ConnectivityClass Connectivity

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GlycO

Class Connectivity

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N-g

lyca

n_be

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Class

BioMedical OntologiesOntology No. of Terms

(Instances)Average No. of

SubtermsConnectivit

y

Protein-protein Interaction

195 4.6 1.1

MGED 228 5.1 0.3

Biological Imaging Methods

260 5.2 1.0

Physico-chemical Process

550 2.7 1.3

Cereal Plant Trait 692 3.7 1.1

BRENDA 2,222 3.3 1.2

Human Disease 19,137 5.5 1.0

Gene Ontology 20,002 4.1 1.4

Conclusions

• More ontologies are introduced as the semantic web is gaining momentum.

• There is no easy way for users to choose the most suitable ontology for their applications.

• OntoQA offers 3 categories of metrics to describe the quality and nature of an ontology.

Future Work

• Calculation of domain dependent metrics that makes use of some standard ontology in a certain domain.

• Making OntoQA a web service where users can enter their ontology files paths and use OntoQA to measure the quality of the ontology.

Questions