In this talk, we focus on the analysis of bibliometric networks, and in particular on the detection of communities in these networks. We start by demonstrating VOSviewer, a popular software tool for visualizing bibliometric networks. We discuss the techniques used by VOSviewer for visualizing bibliometric networks and for detecting communities in these networks. We pay special attention to the close relationship between visualization and community detection, and we discuss the unified approach to visualization and community detection that is implemented in VOSviewer. We then shift our attention to community detection in very large citation networks, including millions of publications and hundreds of millions of citation relations. We show how community detection techniques can be used to construct highly detailed classification systems of science. We also discuss applications of such classification systems to science policy questions. Finally, we demonstrate CitNetExplorer, a new software tool in which community detection techniques are used to support the large-scale analysis of citation networks. We use CitNetExplorer to analyze the citation network of publications on network science and in particular on community detection.

1. Applications of community detection in
bibliometric network analysis
Nees Jan van Eck
Centre for Science and Technology Studies (CWTS), Leiden University
EURANDOM workshop “Networks with community structure”, Eindhoven
January 24, 2014

2. Outline
• Bibliometric network analysis at CWTS
• VOSviewer
• Unified approach to visualization and community
detection
• Community detection in large citation networks
• CitNetExplorer
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3. Bibliometric
network analysis
at CWTS
2

4. Bibliometric network analysis at CWTS
• In-house databases:
– Thomson Reuters Web of Science
– Elsevier Scopus
• Bibliometric networks:
– Publication citation networks
– Journal co-citation/bibliographic coupling networks
– Term co-occurrence networks
– Co-authorship networks
– Etc.
• Applications:
– Research institutions: Research assessment
– Scientific publishers: Journal profiling
– Funding agencies: Science policy analyses
3

5. VOSviewer
(www.vosviewer.com)
4

6. VOSviewer
(Van Eck & Waltman, Scientometrics, 2010)
5

7. Citation network of fields in Web of
Science
6

8. Co-occurrence network of terms in
clinical neurology
7

9. Unified
approach to
visualization and
community
detection
8

10. Visualization vs. community detection
• Visualization (‘mapping’):
– Assigning the nodes in a network to locations in a (usually twodimensional) space
• Community detection (‘clustering’):
– Partitioning the nodes in a network into a number of groups
9

11. Community detection seen as
visualization in a restricted space
10

12. Community detection seen as
visualization in a restricted space
11

13. Unified approach to visualization and
community detection
Minimize Q (x 1 , , x n )
i j
2m
Aij d ij2
kik j
d ij
i j
where
n: number of nodes in the network
m: total weight of all edges in the network
Aij: weight of edge between nodes i and j
ki: total weight of all edges of node i
Visualization
Community detection
xi: integer denoting the community
to which node i belongs
0
if x i x j
d ij
1
if x i x j
: resolution parameter
xi: vector denoting the location of
node i in a p-dimensional
space
p
d ij
xi
xj
(x ik
x jk )2
k 1
12

14. Unified approach: Community detection
Equivalent to a weighted variant of modularity-based
community detection (Waltman et al., 2010)
Maximize
ˆ
Q(x 1 , , x n )
1
2m
(x i , x j )w ij Aij
i j
kik j
2m
where
(xi, xj) equals 1 if xi = xj and 0 otherwise
w ij
2m
kik j
13

15. Unified approach: Visualization
• Equivalent to the VOS (visualization of similarities)
technique (Van Eck & Waltman, 2007)
• Limit case of multidimensional scaling (Van Eck et
al., 2010)
Q
i j
2m
Aij x i
kik j
Wij Dij
xi
xj
2
xi
xj
VOS
i j
xj
2
MDS
i j
Dij
kik j
2m
Aij
1
Wij
2m
Aij
kik j
14

16. Unified approach
Most commonly used community detection technique
(modularity) and most commonly used visualization
technique (MDS) can be brought together in a unified
framework
Unified
approach
Modularity
(weighted)
VOS
MDS
(limit case)
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17. Community
detection in
large citation
networks
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18. Classification systems of scientific
publications
• Web of Science/Scopus classification systems:
– Scientific fields defined at the level of journals rather than individual
publications
– Difficulties with multidisciplinary journals
– High level of aggregation
– Sometimes outdated or inaccurate
• Disciplinary classification systems:
– E.g., CA, JEL, MeSH, PACS
– Not available for all disciplines
– Sometimes outdated or inaccurate
17

19. Algorithmically constructed
classification systems
• Publications (not journals) are clustered into fields based
on citation relations
• Fields are defined at different levels of granularity and
are organized hierarchically
• Community detection based on a variant of the standard
modularity function that accounts for differences in
citation practices across fields
• Optimization using the smart local moving algorithm
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20. Example (Waltman & Van Eck, 2012)
• 10.2 million publications from the period 2001–2010
indexed in Web of Science
• 97.6 million citation relations
• Classification system of 3 hierarchical levels:
– 20 broad disciplines
– 672 fields
– 22,412 subfields
19

21. Visualization of 672 research areas at
level 2 of the classification system
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22. Visualization of 417 publications in
research area 4.30.10
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23. Application in a science policy context
22

24. CitNetExplorer
(www.citnetexplorer.nl)
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25. Exploring citation networks
• Macro-level applications:
– Studying the development of a research field over time
– Identifying research areas
• Micro-level applications:
– Studying the publication oeuvre of a researcher
– Supporting systematic literature reviewing
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26. HistCite
• Timeline visualization of publications and their citation
relations, referred to as algorithmic historiography by
Eugene Garfield
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27. CitNetExplorer
• New software tool for analyzing and visualizing citation
networks
• Freely available on www.citnetexplorer.nl
• Runs on any system that offers Java support
• Citation networks can be constructed directly based on
data downloaded from Web of Science
• Interactive functionality for drilling down into a citation
network
• Very large citation networks can be handled, with
millions of publications and tens of millions of citation
relations
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28. Demonstration
• Database: Web of Science
• Fields: Physics and multidisciplinary (Nature, PLoS
ONE, PNAS, Science, etc.)
• Time period: 1998–2012
• Number of publications: ~1.8 million
• Number of citation relations: ~15.1 million
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29. CitNetExplorer
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30. References
Van Eck, N.J., & Waltman, L. (2010). Software survey: VOSviewer, a computer program for
bibliometric mapping. Scientometrics, 84(2), 523-538.
Van Eck, N.J., & Waltman, L. (2011). Text mining and visualization using VOSviewer. ISSI
Newsletter, 7(3), 50-54.
Van Eck, N.J., Waltman, L., Dekker, R., & Van den Berg, J. (2010). A comparison of two techniques for
bibliometric mapping: Multidimensional scaling and VOS. JASIST, 61(12), 2405-2416.
Waltman, L., & Van Eck, N.J. (2012). A new methodology for constructing a publication-level
classification system of science. JASIST, 63(12), 2378-2392.
Waltman, L., & Van Eck, N.J. (2013). A smart local moving algorithm for large-scale modularity-based
community detection. European Physical Journal B, 86(11), 471.
Waltman, L., Van Eck, N.J., & Noyons, E.C.M. (2010). A unified approach to mapping and clustering of
bibliometric networks. Journal of Informetrics, 4(4), 629-635.
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