| Independent variables | Symbols | Coded variables | ||
|
|
| −1 | 0 | 1 |
| Yam flour (g) | X1 | 40 | 45 | 50 |
| Herring meal (g) | X2 | 10 | 15 | 20 |
| Pigeon pea flour (g) | X3 | 20 | 25 | 30 |