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Variables and experimental design levels for response surface.
| Independent Variables | Model | Symbol Coded Factor Level | ||||
|---|---|---|---|---|---|---|
| −2 | −1 | 0 | +1 | +2 | ||
| Extraction time (h) | X1 | 1.5 | 2 | 2.5 | 3 | 3.5 |
| Extraction temperature (°C) | X2 | 45 | 50 | 55 | 60 | 65 |
| Extraction pressure (MPa) | X3 | 10 | 15 | 20 | 25 | 30 |
| Ethanol concentration (%) | X4 | 80 | 85 | 90 | 95 | 100 |
