Table 1 Coded and uncoded values of factors in the Plackett-Burman design

X₁

(NH₄)₂SO₄ (%, w/w)

0.5

1.0

1.5

X₂

Initial moisture content (%, w/w)

45

50

55

X₃

CaCl₂ (%, w/w)

0.1

0.2

0.3

X₄

MgCl₂ (%, w/w)

0.050

0.075

0.100

X₅

KH₂PO₄ (%, w/w)

0.050

0.0625

0.075

X₆

ZnSO₄ (%, w/w)

0.05

0.075

0.10

X₇

MnSO₄ (%, w/w)

0.05

0.075

0.10

X₈

FeSO₄ (%, w/w)

0.05

0.075

0.10

X₉ – X₁₁

Dummy variables