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Table 1 Coded levels for variables used in the experimental design

From: Optimization of algal methyl esters using RSM and evaluation of biodiesel storage characteristics

Factors Symbol Coded levels
−1 0 +1
Oil:alcohol ratio (v/v) A 4 6 8
Catalyst amount (w/w) B 0.1 0.3 0.5
Time (min) C 30 90 150
Temperature (° C) D 50 70 90