Mapping Science - Mapping Scientific Frontiers: The Quest for Knowledge Visualization - page 273

Tabl e 8. 2 Major clusters of co-cited references
Cluster ID Size Silhouette Label (TFIDF)

Label (LLR) Label (MI) Year Ave.

9 97 0.791 Evolving concept Mesenchymal stem

cell

Cardiac
progenitor
cell

1999

17 71 0.929 Somatic control Drosophila

spermatogenesis

Drosophila 1994

6 67 0.980 Mcf-7 cell Intestinal-type

gastric cancer

Change 2001

12 62 0.891 Midkine Human embryonic

stem cell

Dna 2002

5 53 0.952 Grid2ip gene Silico Gastric cancer 2002

19 42 0.119 Bevacizumab Combination Cartilage 2004

7 40 0.960 Monogenic disease

treatment

Induced pluripotent
stem cell

Clinic 2008

15 25 0.930 Tumorigenic

melanoma cell

Cancer stem cell Cancer

prevention

2003

Clusters are referred in terms of the labels selected by LLR

followed by #20 human embryonic stem cell, and then followed by the latest and
current #32 induced pluripotent stem cell. The patches of red rings in #32 indicate
this area is rapidly expanding as suggested by citation bursts.
Tab le 8.2 lists eight major clusters by their size, i.e. the number of members in
each cluster. Clusters with few members tend to be less representative than larger
clusters because small clusters are likely to be formed by the citing behavior of
a small number of publications. The quality of a cluster is also reflected in terms
of its silhouette score, which is an indicator of its homogeneity or consistency.
Silhouette values of homogenous clusters tend to close to 1. Most of the clusters
are highly homogeneous, except Cluster #19 with a low silhouette score of 0.119.
Each cluster is labeled by noun phrases from titles of citing articles of the cluster
(Chen et al. 2010 ).
The average year of publication of a cluster indicates its recentness. For example,
Cluster #9 on mesenchymal stem cell (MSCs) has an average year of 1999. The most
recently formed cluster, Cluster #7 on induced pluripotent stem cell (iPSCs), has an
average year of 2008.
Cluster #7 contains numerous nodes with red rings of citation bursts. The
visualized network also shows highly burst terms found in the titles and abstracts
of citing articles to the major clusters. For example, terms stem - cell - renewal and
germ - line - stem - cells are not only used when articles cite references in Cluster #17
drosophila spermatogenesis, but also used with a period of rapid increase. Similarly,
the term induced - pluripotent - stem - cells is a burst term associated with Cluster
#7, which is consistently labeled as induced pluripotent stem cell by a different
selection mechanism, the log-likelihood ratio test (LLR). We will particularly focus
on Cluster #7 in order to identify emerging trends in regenerative medicine.
Cluster #7 is the most recently formed cluster. We selected ten most cited
references in this cluster and 10 citing articles (See Table 8.3 ).