Abstract:
In this paper the difference of the variance functions between
two estimated responses for a fourth order design at any two points in the
factor space is developed. In particular, the variance function is considered
in two dimensions when the design used is rotatable. The variance function
in this situation is a function of the distances of the points from the origin
of the design and the angle subtending the points at the origin. The
variance function of this approach is discussed in detail when the two
points are equidistant from the origin of the design. The criterion for the
choice of an optimal design is given
