where ε represents the noise or error observed in the response y. The surface
represented by f(x1, x2) is called a response surface.
The response can be represented graphically, either in the three-dimensional
space or as contour plots that help visualize the shape of the response surface.
Contours are curves of constant response drawn in the xi
, xj
plane keeping all other
variables fixed. Each contour corresponds to a particular height of the response
surface, as shown in Figure 3.1.