alpha determines whether the star points lie on the
faces of the cube (alpha=1, face-centered), inside the cube (alpha<1< code="">,
inscribed) or outside the cube (alpha>1, circumscribed).
The latter case is the usual one. The value of
alpha can be chosen such that the design is rotatable (may be useful if the scales
of the factors are comparable) or such that the design is orthogonally blocked
(i.e. the block effects do not affect the effect estimates of interest). The default
is to generate orthogonally blocked designs.
Central composite designs are particularly useful in sequential experimentation,
where a (fractional) factorial with center points is followed up by a star portion
of the design. While the cube can already estimate the linear and interaction effects,
the center points can only estimate the sum of all quadratic effects. If this indicates
that quadratic effects are important, a star portion can be added in order to investigate
the model more deeply.
ccd, ccd.design, ccd.augment