However, by augmenting a minimum run-size DSD with four or more properly selected runs, you can identify substantially more effects with high probability. These runs are called Extra Runs, and correspond to fictitious inactive factors, called fake factors. For information about Extra Runs, see Structure of Definitive Screening Designs in Definitive Screening Designs.
Extra Runs substantially increase the design’s ability to detect second-order effects. For this reason, Jones and Nachtsheim (2016) strongly encourage the inclusion of at least four Extra Runs.