Independent variable Symbol Coded levels
−2  −1  0  +1 +2
Natural level
pH A 2.5 5 7.5 10 12.5
Tetracycline (mg/L) B 10 30 50 70 90
Persulfate (Mm) C 1 2 3 4 5
Reaction time (min) D 30 60 90 120 150