Run Independent values Hydrogen production (ml L-1) Categorical factor levels Coded values Real values Observed Predicted   No. X1 X2 X3 X1 X2 X3       1 +1 +1 +1 12 200 500 650 644.43 Fractional 23 fractional factorial points 2 +1 +1 -1 12 200 300 700 702   3 +1 -1 -1 12 100 300 600 599.63   4 +1 -1 +1 12 100 500 575 584.4   5 -1 +1 -1 8 200 300 520 517.56   6 -1 +1 +1 8 200 500 640 647.43   7 -1 -1 -1 8 100 300 360 372.53   8 -1 -1 +1 8 100 500 540 544.81   9 +α 0 0 13.05 150 400 850 849.46 Star point (6 points) 10 - α 0 0 6.95 150 400 690 678.56   11 0 +α 0 10 226.23 400 540 535.35   12 0 -α 0 10 73.76 400 600 585.85   13 0 0 +α 10 150 552.46 630 622.67   14 0 0 -α 10 150 247.54 540 544.81   15 0 0 0 10 150 400 960 987.02 Central points (6 points) 16 0 0 0 10 150 400 992 987.02   17 0 0 0 10 150 400 990 987.02   18 0 0 0 10 150 400 993 987.02   19 0 0 0 10 150 400 994 987.02   20 0 0 0 10 150 400 993 987.02

Table 2: Central composite experimental design with three independent variables.