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Table 4 Coded values of the variables for the central composite design

From: Photo-biohydrogen production potential of Rhodobacter capsulatus- PK from wheat straw

   Actual values of coded levels   
Variables Coded symbol -1 0 1 -1.6818 1.6818
-Inoculum age (h) X1 24 48 72 8.00 88
Nitrogen content (mg) X2 200 300 400 132 468
Substrate (g) X3 1.5 2.0 2.5 1.16 2.84