In some circumstances, a complete description of the process behaviour might require a
quadratic or cubic model.
A typical quadratic model is of the form:
Y ¼ b0 þ b1X1 þ b2X2 þ b3X3 þ b12X1X2
þb13X1X3 þ b23X2X3 þ b11X2
1 þ b22X2
2 þ b33X2
3;
ð2Þ
While a typical cubic function will be of the form:
^Y
¼ Quadratic model þ b123X1X2X3 þ b112X2
1X4 þ b113X2
1X3
þb112X1X2
3 þ b223X2
2X3 þ b233X2X2
3 þ b111X3
1
þb222X3
2 þ b333X2
3
ð3Þ
In a more general case of a response surface polynomial function of n independent
variables, (1) becomes:
Y ¼ b0 þ b1X1 þ b2X2 þ . . . þ bnXn
þb11X2 þ b12X1X3 þ b13X1X3 þ b1; 4X1X4 þ . . . þ b1; nX1Xn
þb22X2
2 þ b2; 3X2X3 þ b3; 4X3X4 þ b3; 5X3X5 þ . . . þ b3; 4X3X4
þb3; 5X3X5 þ b3; 6X3X6 þ . . . þ b3; nX3Xn
þ.. .
þbmmXm
m þ bm;mþ1Xmþ1
þb1; 2; 3X1X2X3 þ b1; 2; 4X1X2X4 þ b1; 2; 5X1X2X5
þ. . . þ b1; 2; nX1X2Xn
þb2; 3; 4X2X3X4 þ b2; 3; 6 þ X2X3X6
þ. . . þ b2; 3; nX2X3Xn
þ.. .
m; m þ 1; m þ 2 ðwherem þ 2 ¼ nÞ þ 1: