Loet Leydesdorff &
Ismael Rafols talk with ScienceWatch.com and
answer a few questions about this month's Fast Breaking
Paper in the field of Social Sciences, general. The
authors have also sent along images of their
Article Title: A Global Map of Science Based on the
ISI Subject Categories
Authors: Leydesdorff, L;Rafols, I
Journal: J AM SOC INF SCI TECHNOL
Year: FEB 2009
* Univ Amsterdam, ASCoR, Kloveniersburgwal 48, NL-1012 CX
* Univ Amsterdam, ASCoR, NL-1012 CX Amsterdam,
* Univ Sussex, Freeman Ctr, Brighton BN1 9QE, E Sussex,
Why do you think your paper is highly
First, it offers a simple and comprehensive map of science
(view map) in terms of 170+ disciplinary categories
based on the aggregated citation patterns of journals included in the
Science Citation Index. This map can be
recognized intuitively in terms of disciplinary delineations and be
accessed by mouse-clicking between different levels of aggregation
(view map): 14 disciplines at the top level, 172
categories at the intermediate level, and 6,164 individual journals at
The delineation of journal sets in terms of disciplines and specialties is
relevant in research evaluation because publication and citation practices
differ among fields of science. Furthermore, the map helps to identify and
locate interdisciplinary research at the borders of or across disciplines.
This is important because, although "interdisciplinarity" is a policy
objective, its assessment has hitherto remained controversial.
Also, this map of science can be used as a baseline for understanding the
cognitive and potentially changing positions of document sets. For example,
one can study and visualize the diffusion of emerging fields like
nanotechnology. Finally, the paper may be cited for the techniques of
clustering large databases.
Does it describe a new discovery, methodology, or
synthesis of knowledge?
The paper offers a synthesis of previous knowledge on the basis of a
methodological consideration. Although the attribution of journals to
subject categories remains highly uncertain—and therefore often
erroneous—we show that the average attribution at the aggregated
level can be used for the mapping of science, because the errors can be
expected to average out—like the noise in pixelated pictures.
Previous efforts to use the complete matrix of aggregated journal-journal
citations for mapping were not as successful because one had to make a
number of assumptions to reduce the data (Leydesdorff, 2006).
A combination of previous investments in the attribution of
journals—by indexers at the Institute for Scientific
Information (ISI), now part of
Thomson Reuters—and relatively straightforward
analytical techniques, enabled us to generate a map from an already
This cannot be considered a discovery because structure has been observed
in the citation matrix based on the
Journal Citation Reports®
(JCR) since its very beginning (Price, 1965). The question was
then how to represent the cognitive structure contained in this textual
Would you summarize the significance of your paper in
Despite considerable uncertainty inherent in the underlying data, the
existing classification of journals enabled us to generate a comprehensive
map of science that shows the structure of science in terms of 14
disciplines and 172 specialties.
As in the case of Google maps, we provide a user-friendly
toolkit to construct an overlay, in order to
indicate the position of publications or citations in terms of their
disciplinary affiliations and their potential interdisciplinary
These "overlay maps" can be used for various research policy or management
purposes. For example, emerging fields can be traced in terms of diffusion
patterns (view map) over the cognitive landscape of the
sciences (Porter & Youtie, 2009: 1029). Science managers can easily
locate the scientific areas in which their organization is most active
or into which they might wish to grow.
How did you become involved in this research, and were
there any problems along the way?
can be recognized intuitively in terms of
disciplinary delineations and be accessed by
mouse-clicking between different
levels of aggregation: 14 disciplines at the
top level, 172 categories at the intermediate
level, and 6,164 individual journals at the
We are both interested in mapping the emergence of new and
interdisciplinary developments (e.g., nanotechnologies, genomics, etc.).
How can interdisciplinary dynamics be indicated at the global level of the
development of the sciences? First, we had to develop an indicator of
betweenness centrality in journal maps (Leydesdorff, 2007), and for the
diversity of the knowledge base of a document set (Rafols & Meyer, in
The first author (Loet Leydesdorff) has been studying journal-journal
citation patterns since the 1980s (e.g., Leydesdorff, 1987). The purpose of
his studies is to use the development of journal structures as a baseline
for indicating change in science at the global level, that is, beyond the
control of individual agency or policy making. Policy interventions can
then be calibrated and therefore evaluated in terms of contributions to
cognitive advancements (Leydesdorff & Schank, 2008).
Ismael Rafols, the second author, focuses on interdisciplinary developments
in the sciences, notably nanotechnology, using both qualitative analyses
and bibliometrics (Rafols & Meyer, 2007). The diffusion of new
developments across disciplinary boundaries, mergers, and bifurcations can
be followed using the proposed technique of an overlay (Rafols & Meyer,
in press). Sets of documents (for example, the output of a laboratory) can
thus be positioned at each moment and over time (Rafols, Porter, &
Where do you see your research leading in the
We have extended this line of research further into a systematic test of
the subject categories used for the map in a follow-up paper entitled
"Content-based and Algorithmic Classifications of Journals: Perspectives on
the Dynamics of Scientific Communication and Indexer Effects,"
JAIST 60(9), 1823-35, 2009. Here we compare decomposition in terms
of subject categories—which are content-based—with
decomposition as indicated by fast computer programs. Our conclusion is
that despite such differences in approach, a "consensus map" of science can
increasingly be retrieved from various possible representations (Klavans
& Boyack, 2009).
The advantages of our map are its intuitive user-friendliness and the
availability of the overlay technique which allows users to adapt it for
their own purposes
(freely available here).
In the longer term, one would like to have dynamic Google-like maps of the
socio-cognitive space of the sciences in which one could trace one's own
fields and/or publications (Boerner & Scharnhorst, 2009; Leydesdorff
& Schank, 2008).
Do you foresee any social or political implications for
In a knowledge-based society and a knowledge-based economy, the
intellectual organization of knowledge in the sciences is increasingly
important because knowledge organizes society together with its
coordination by the market and (largely national) political systems.
People connect because of knowledge-based professional ties. Policy
interventions often focus on the political economy but not sufficiently on
the knowledge structures that change our systems in waves of
knowledge-based innovations. The mapping of the origins and contents of
these waves may contribute to the shaping of future wealth from knowledge.
Loet Leydesdorff, Ph.D.
University of Amsterdam
Amsterdam School of Communication Research (ASCoR)
Ismael Rafols, Ph.D.
SPRU - Science and Technology Policy Research
University of Sussex
Boerner, K; Scharnhorst, A, "Visual conceptualizations and
models of science," Journal of Informetrics, 3, 161-72,
Klavans, R; Boyack, K, "Towards a Consensus Map of Science,"
JASIST 60, 455-76, 2009.
Leydesdorff, L, "Various methods for the Mapping of
Science," Scientometrics 11, 291-320, 1987.
Leydesdorff, L, "Can Scientific Journals be Classified in
Terms of Aggregated Journal-Journal Citation Relations using the Journal
Citation Reports?," JASIST 57, 601-13, 2006.
Leydesdorff, L, " 'Betweenness Centrality' as an Indicator
of the 'Interdisciplinarity' of Scientific Journals," JASIST
58, 1303-09, 2007.
Leydesdorff, L; Schank, T, "Dynamic Animations of Journal
Maps: Indicators of Structural Change and Interdisciplinary Developments"
JASIST 59, 1810-18, 2008.
Porter, AL; Youtie, J, "How interdisciplinary is
nanotechnology?" Journal of Nanoparticle Research, 11,
Price, D. J. de Solla, "Networks of scientific papers,"
Science 149, 510-15, 1965.
Rafols, I; Leydesdorff, L, "Content-based and Algorithmic
Classifications of Journals: Perspectives on the Dynamics of Scientific
Communication and Indexer Effects," JASIST 60, 1823-35,
Rafols, I; Meyer, M, "How cross-disciplinary is
bionanotechnology? Explorations in the specialty of molecular motors,"
Scientometrics 70, 633-50, 2007.
Rafols, I; Meyer, M (in press), "Diversity and Network
Coherence as indicators of interdisciplinarity: case studies in
bionanoscience" Scientometrics, DOI
Rafols, I; Porter, AL; Leydesdorff, L, "Science Overlay
Maps: A New Tool for Research Evaluation," Paper presented at the Atlanta
Conference on Science and Innovation Policy, October 1-2, 2009, Atlanta,
KEYWORDS: JOURNAL-CITATION-REPORTS; SCIENTIFIC JOURNALS;
INDICATORS; INTERDISCIPLINARITY; COCITATION; DECOMPOSABILITY;
CLASSIFICATION; AGGREGATION; PERFORMANCE; RELEVANCE.