Table 1. Range and level of parameters in the design
Variables
Levels
−2
−1
0
+1
+2
Temperature (°C) (X
1
)
45
60
75
90
105
Ratio 1 (X
2
)
0.5
2
3.5
5
6.5
Ratio 2 (X
3
)
1.5
3
4.5
6
7.5
Time (hour) (X
4
)
1
3
5
7
9