Mapping Science - Mapping Scientific Frontiers: The Quest for Knowledge Visualization - page 276

Tabl e 8. 5 References with the strongest citation bursts
Citation bursts

References Cluster #

124.73 Takahashi K, 2006, Cell, v126, p663 7

121.36 Takahashi K, 2007, Cell, v131, p861 7

81.37 Yu JY, 2007, Science, v318, p1917 7

71.24 Okita K, 2008, Science, v322, p949 7

66.23 Meissner A, 2008, Nature, v454, p766 13

63.12 Vierbuchen T, 2010, Nature, v463, p1035 8

62.54 Zhou HY, 2009, Cell Stem Cell, v4, p381 7

Tabl e 8. 6 Structurally and temporally significant references

Sigma Burst Centrality Citations References Cluster #

377340.46 124.73 0.11 1,841 Takahashi K, 2006, Cell, v126, p663 7

29079.18 37.38 0.32 202 Bjornson CRR, 1999, Science, v283, p534 9

195.15 121.36 0.04 1,583 Takahashi K, 2007, Cell, v131, p861 7

58.91 81.37 0.05 1,273 Yu JY, 2007, Science, v318, P1917 7

15.97 19.53 0.15 130 Kiger AA, 2000, Nature, v407, p750 17

in Cluster #7) and one 2010 article (in Cluster #8, a small cluster) are detected to
have considerable degrees of citation burst. The leader of the group that authored
the top two references was awarded the 2012 Nobel Prize in Medicine.
The Sigma metric measures both structural centrality and citation burstness of a
cited reference. If a reference is strong in both measures, it will have a higher Sigma
value than a reference that is only strong in one of the two measures.
AsshowninTable 8.6 , the pioneering iPSCs article by Takahashi (2006) has
the highest Sigma of 377340.46, which means it is structurally essential and
inspirational in terms of its strong citation burst. The second highest work by this
measure is a 1999 article in Science by Bjornson et al. ( 1999 ). They reported an
experiment in which neural stem cells were found to have a wider differentiation
potential than previously thought because they evidently produced a variety of blood
cell types.

8.2.3 System-Level Indicators

The modularity of a network measures the degree to which nodes in the network
can be divided into a number of groups such that nodes within the same group are
connected tighter than nodes between different groups. The collective intellectual
structure of the knowledge of a scientific field can be represented as associated
networks of co-cited references. Such networks evolve over time. Newly published
articles may introduce profound structural variation or have little or no impact on
the structure.