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Factors and levels in response surface methodology (RSM)
| Independent variable | Symbol | Level | ||
|---|---|---|---|---|
| −1 | 0 | 1 | ||
| Ethanol concentration (%) | X 1 | 35 | 62.5 | 90 |
| Solvent‐to‐material ratio (ml/g) | X 2 | 15 | 22.5 | 35 |
| Ultrasonic time (min) | X 3 | 15 | 35 | 55 |
| Ultrasonic power (W) | X 4 | 100 | 300 | 500 |
