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Folding Over a Design

Design folding is a technique used to increase the resolution of a Resolution 3 two-level fractional factorial design by adding runs that are taken from the fraction opposite the original fraction of the corresponding full factorial design. In other words, for each run in the original design, a run is added in which the coded factor levels are switched in sign. Fully folding over a design doubles the number of runs.

This technique is recommended if you notice the potential significance of two-factor interactions but are still interested only in the main effects. Since folding over increases the resolution to 4, the main effects in the augmented design can be estimated independently of the two-factor interactions. You can select any or all factors to fold over.