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. 2019 Feb 17;7(3):937–948. doi: 10.1002/fsn3.880

Table 1.

Range of values for response surface methodology

Independent variables Variable name Levelsa
−2 −1 0 1 2
X 1 (ml/L) Waste cooking oil amount 54.8 65.0 80.0 95.0 105.2
X 2 (ml/L) Inoculum size 11.6 15.0 20.0 25.0 28.4
X 3 (ml/250 ml) Medium volume 24.77 35.00 50.00 65.00 75.23
X 4 Initial pH 3.41 4.43 5.93 7.43 8.45
a

x 1 = (X 1 − 80)/15; x 2 = (X 2 − 20)/5; x 3 = (X 3 − 50)/15; x 4 = (X 4 – 5.93)/1.5.