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Plackett-Burman range and the levels of the variables.
| Independent variable (X i) | Range and levels | |
|---|---|---|
| −α | +α | |
| Lactose (%) | 8 | 12 |
| Glucose (%) | 8 | 12 |
| Dye concentration (ppm) | 200 | 1000 |
| Ammonium sulphate (%) | 9 | 13 |
| Ammonium nitrate (%) | 9 | 13 |
| Tryptone (%) | 9 | 13 |
| Yeast (%) | 9 | 13 |
| Ammonium chloride (%) | 9 | 13 |
| Inoculum size (cm) | 0.2 | 1.0 |
| Moisture content (%) | 40 | 80 |
| Incubation time (days) | 3 | 15 |
