Optimal Foldover Plans for Mixed-level Fractional Factorial Designs

Reversing plus and minus signs of one or more factors is the traditional method to fold over two-level fractional factorial designs. However, when factors in the original design have more than two levels, the method of 'reversing signs' loses its efficacy. This article develops a mechanism to foldover designs involving factors with different numbers of levels, say mixed-level designs. By exhaustive search we identify the optimal foldover plans. The criterion used is the general balance metric, which can reveal the aberration properties of the combined designs (original design plus joldover). The optimal foldovers for some efficient mixed-level fractional factorial designs are provided for practical use. Copyright (C) 2008 John Wiley & Sons, Ltd.