Table 8.

Topic clusters according to cited documents.

Cluster	Size	Mean Silhouette	Label (TFIDF)	Label (LLR)	Mean (Year)
0	88	0.708	low-cost carriers	low-cost carrier (124.57, 1.0E-4)	2012
1	66	0.690	Taiwan	low-cost airline (96.56, 1.0E-4)	2012
2	65	0.836	making approach	airline service quality (108.43, 1.0E-4)	2014
3	59	0.883	efficiency	small regional airport sustainability (76.91, 1.0E-4)	2010
4	55	0.847	high-speed rail transport integration	aviation research data (72.57, 1.0E-4)	2014
5	53	0.859	evolution	strategic alliance (89.56, 1.0E-4)	2001
6	53	0.716	airports	analytic hierarchy process assessment (83.44, 1.0E-4)	2012