method

2014

2015

2016

2017

The empirical research

5

7

6

3

Normative analysis

4

5

0

0

The investigation and study

2

3

4

2

The case study

2

2

3

2

Analytical model research

5

2

2

4

Experimental study

0

0

0

0

Literature research

0

1

0

0

Total

18

20

15

11