Semifold plans for mixed-level designs

This article presents a more efficient method for sequential augmentation of mixed-level designs. The proposed approach reduces the optimal foldover plan of a mixed-level design to a semifold plan by selecting half of the treatment combinations of the foldover fraction using exhaustive search and the criterion of general balance metric. The resulting design is a more economic run size augmented fraction that possesses good balance and orthogonality properties for main effects and two-factor interactions. Three efficient arrays consisting of 20, 24 and 30 runs were selected for the analysis. Efficient arrays composed of a higher number of runs can be semifolded in a similar manner.