| Variable | Symbol | Coded level | ||
| −1 | 0 | +1 | ||
| Time (mins) | X1 | 40 | 80 | 120 |
| NaOH (g) | X2 | 1.0 | 1.3 | 1.6 |
| Molar ratio (methanol/palm oil) | X3 | 3.6 | 6 | 8.4 |