Table 1 Overview of issuing authors.

Volume of literature (articles)

6

59

152

Nodes (pcs)

4

61

170

Frequency (times)

7

71

210

Node/frequency ratio

0.57

0.86

0.81

Standard deviation of frequencies

1.5

0.61

1.55

Co-occurrence (times)

1

6

12

Co-occurrence/node ratio

0.14

0.08

0.06