Table 7.

Hierarchical structure of influencing factors.

Hierarchy	Factors
Level 1(Top level)	a5,a6,a8,a14,a20
Level 2	a9
Level 3	a3,a4,a10,a13,a19
Level 4	a7,a16
Level 5	a11,a12,a15,a18
Level 6	a17
Level 7(Bottom level)	a1,a2