Table 2. Network statistics of the keyword co-occurrence networks.

Network Measures	1993–1997	1998–2002	2003–2007	2008–2012	Total in 20 years
Edges	7656	15542	24922	38345	58532
Average weighted degree	25.052	59.520	113.944	205.934	404.460
Network density	0.015	0.031	0.050	0.077	0.117
Modularity	0.325	0.320	0.323	0.276	0.289
Average clustering co-efficient	0.525	0.491	0.446	0.455	0.477