5,904 research outputs found
Multi-level algorithms for modularity clustering
Modularity is one of the most widely used quality measures for graph
clusterings. Maximizing modularity is NP-hard, and the runtime of exact
algorithms is prohibitive for large graphs. A simple and effective class of
heuristics coarsens the graph by iteratively merging clusters (starting from
singletons), and optionally refines the resulting clustering by iteratively
moving individual vertices between clusters. Several heuristics of this type
have been proposed in the literature, but little is known about their relative
performance.
This paper experimentally compares existing and new coarsening- and
refinement-based heuristics with respect to their effectiveness (achieved
modularity) and efficiency (runtime). Concerning coarsening, it turns out that
the most widely used criterion for merging clusters (modularity increase) is
outperformed by other simple criteria, and that a recent algorithm by Schuetz
and Caflisch is no improvement over simple greedy coarsening for these
criteria. Concerning refinement, a new multi-level algorithm is shown to
produce significantly better clusterings than conventional single-level
algorithms. A comparison with published benchmark results and algorithm
implementations shows that combinations of coarsening and multi-level
refinement are competitive with the best algorithms in the literature.Comment: 12 pages, 10 figures, see
http://www.informatik.tu-cottbus.de/~rrotta/ for downloading the graph
clustering softwar
Construction of a Pragmatic Base Line for Journal Classifications and Maps Based on Aggregated Journal-Journal Citation Relations
A number of journal classification systems have been developed in
bibliometrics since the launch of the Citation Indices by the Institute of
Scientific Information (ISI) in the 1960s. These systems are used to normalize
citation counts with respect to field-specific citation patterns. The best
known system is the so-called "Web-of-Science Subject Categories" (WCs). In
other systems papers are classified by algorithmic solutions. Using the Journal
Citation Reports 2014 of the Science Citation Index and the Social Science
Citation Index (n of journals = 11,149), we examine options for developing a
new system based on journal classifications into subject categories using
aggregated journal-journal citation data. Combining routines in VOSviewer and
Pajek, a tree-like classification is developed. At each level one can generate
a map of science for all the journals subsumed under a category. Nine major
fields are distinguished at the top level. Further decomposition of the social
sciences is pursued for the sake of example with a focus on journals in
information science (LIS) and science studies (STS). The new classification
system improves on alternative options by avoiding the problem of randomness in
each run that has made algorithmic solutions hitherto irreproducible.
Limitations of the new system are discussed (e.g. the classification of
multi-disciplinary journals). The system's usefulness for field-normalization
in bibliometrics should be explored in future studies.Comment: accepted for publication in the Journal of Informetrics, 20 July 201
ELASTICITY: Topological Characterization of Robustness in Complex Networks
Just as a herd of animals relies on its robust social structure to survive in
the wild, similarly robustness is a crucial characteristic for the survival of
a complex network under attack. The capacity to measure robustness in complex
networks defines the resolve of a network to maintain functionality in the
advent of classical component failures and at the onset of cryptic malicious
attacks. To date, robustness metrics are deficient and unfortunately the
following dilemmas exist: accurate models necessitate complex analysis while
conversely, simple models lack applicability to our definition of robustness.
In this paper, we define robustness and present a novel metric, elasticity- a
bridge between accuracy and complexity-a link in the chain of network
robustness. Additionally, we explore the performance of elasticity on Internet
topologies and online social networks, and articulate results
Bibliometric Perspectives on Medical Innovation using the Medical Subject Headings (MeSH) of PubMed
Multiple perspectives on the nonlinear processes of medical innovations can
be distinguished and combined using the Medical Subject Headings (MeSH) of the
Medline database. Focusing on three main branches-"diseases," "drugs and
chemicals," and "techniques and equipment"-we use base maps and overlay
techniques to investigate the translations and interactions and thus to gain a
bibliometric perspective on the dynamics of medical innovations. To this end,
we first analyze the Medline database, the MeSH index tree, and the various
options for a static mapping from different perspectives and at different
levels of aggregation. Following a specific innovation (RNA interference) over
time, the notion of a trajectory which leaves a signature in the database is
elaborated. Can the detailed index terms describing the dynamics of research be
used to predict the diffusion dynamics of research results? Possibilities are
specified for further integration between the Medline database, on the one
hand, and the Science Citation Index and Scopus (containing citation
information), on the other.Comment: forthcoming in the Journal of the American Society for Information
Science and Technolog
Global Maps of Science based on the new Web-of-Science Categories
In August 2011, Thomson Reuters launched version 5 of the Science and Social
Science Citation Index in the Web of Science (WoS). Among other things, the 222
ISI Subject Categories (SCs) for these two databases in version 4 of WoS were
renamed and extended to 225 WoS Categories (WCs). A new set of 151 Subject
Categories (SCs) was added, but at a higher level of aggregation. Since we
previously used the ISI SCs as the baseline for a global map in Pajek (Rafols
et al., 2010) and brought this facility online (at
http://www.leydesdorff.net/overlaytoolkit), we recalibrated this map for the
new WC categories using the Journal Citation Reports 2010. In the new
installation, the base maps can also be made using VOSviewer (Van Eck &
Waltman, 2010).Comment: Scientometrics, in pres
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