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Coded and uncoded levels of the three independent variables
| Independent variables | Low | Center | High |
|---|---|---|---|
| X1 (ethanol concentration) | − 1 (50%) | 0 (70%) | + 1 (90%) |
| X2 (solvent to sample ratio) | − 1 (10:1) | 0 (25:1) | + 1 (40:1) |
| X3 (extraction time) | − 1 (30 min) | 0 (45 min) | + 1 (60 min) |
