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Independent variables and their levels used in the response surface design.
| Serial number | Independent variables | Symbols | Factor level | ||
|---|---|---|---|---|---|
| Low (−1) | Middle (0) | High (+1) | |||
| 1 | Extraction temperature (°C) | X 1 | 30 | 40 | 50 |
| 2 | Extraction time (hours) | X 2 | 16 | 32 | 48 |
| 3 | Extraction cycles (cycle) | X 3 | 4 | 12 | 20 |
