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Independent variables and their levels in central composite circumscribed (CCC) design.
| Symbol | Independent variable | Factor level | ||||
|---|---|---|---|---|---|---|
| −1.68 | −1 | 0 | 1 | 1.68 | ||
| X 1 | Extraction time (min) | 13 | 20 | 30 | 40 | 47 |
| X 2 | Solid/liquid ratio (g/mL) | 1 : 03 | 1 : 10 | 1 : 20 | 1 : 30 | 1 : 37 |
| X 3 | Extraction temperature (°C) | 33 | 40 | 50 | 60 | 67 |
