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Table 1 Factors and value levels used in the central composite design

From: Recombinant d-galactose dehydrogenase partitioning in aqueous two-phase systems: effect of pH and concentration of PEG and ammonium sulfate

Factors Factor code Inferior levela(−1) Center pointa(0) Superior levela(+1)
PEG (%, w/w) X 1 13 14 15
Salt (%, w/w) X 2 11 12 13
pH X 3 7 7.5 8
  1. aLevel 0 represents the central level of each factor in the intended ranges to evaluate the background variability in the process; levels −1 and +1 are the factorial points, i.e., the high and low levels of each factor in the intended range.