The idle column method, proposed by Taguchi, enables a two-level orthogonal array to be used as the basis of a mixed-level fractional factorial experiment. The resulting designs are not orthogonal but can in some circumstances be described as nearly orthogonal. The method must be applied with care, because an apparently minor change in the technique produces a different kind of design. We illustrate these points with a simple example. The same example is used to compare the “orthogonal” analysis of nonorthogonal designs, as advocated by Taguchi, with the least squares method. Finally, we examine some alternative designs and try to identify the circumstances in which the idle-column method is likely to be most useful.