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Independent variables and levels used for surface response composite central design.
| Independent Variables | Levels | ||||
| −14.142 | −1 | 0 | 1 | 14.142 | |
| OE concentration (%) (X1) | 3.964 | 5 | 7.5 | 10 | 11.035 |
| Plasticizer concentration (%) (X2) | 13.786 | 20 | 35 | 50 | 56.213 |
