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Independent variables and their levels and corresponding coded values used in CCD.
| Independent Variables | Independent Levels | ||||
|---|---|---|---|---|---|
| −2 | −1 | 0 | 1 | 2 | |
| Solvent concentration, X1 (%) | 50 | 60 | 70 | 80 | 90 |
| Material-to-liquid ratio, X2 (g/mL) | 1:5 | 1:15 | 1:20 | 1:25 | 1:30 |
| Extraction temperature, X3 (°C) | 30 | 40 | 50 | 60 | 70 |
| Sonication power, X4 (W) | 180 | 210 | 240 | 270 | 300 |
