Table 3.

Properties of word co-occurrence networks by time series.

Properties		Period 1 (2008-2010)	Period 2 (2011-2013)	Period 3 (2014-2016)	Period 4 (2017-2019)
Word co-occurrence network information, n
	Links in the original network	184,550	414,721	1,084,449	1,576,645
	Links in the extracted network 1 (by the top 100 words in degree centrality values)	4053	4705	4943	4931
	Links in the extracted network 2 (by link reduction)	236	302	318	380
	Threshold of co-occurrence frequencies for link reductiona	7	10	25	12
Network degree centralization, index %		30.2	23.7	47.1	34.4
Distribution of degree centrality
	Mean of degree centrality	0.048	0.061	0.064	0.077
	Standard deviation	0.074	0.072	0.079	0.077
	Min. of degree centrality	0.000	0.000	0.000	0.000
	Max. of degree centrality	0.343	0.293	0.525	0.414

^aThe threshold value was decided by a link reduction simulation, identifying the point where the network component number suddenly increased.