• For continuous factors, the number of required runs is only one more than twice the number of factors. Categorical factors require two more than twice the number of factors.
 • Unlike resolution IV designs, two-factor interactions are not completely confounded with other two-factor interactions, although they might be correlated.
 • Unlike resolution III, IV, and V designs with added center points, all quadratic effects are estimable in models comprised of any number of linear and quadratic main-effects terms.
 • Quadratic effects are orthogonal to main effects and not completely confounded (though correlated) with interaction effects.
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Definitive Screening Design for Eight Three-Level Factors shows an example of a definitive design with eight three-level factors. Notice that each pair of rows is a foldover pair; each even-numbered row mirrors the previous row. This foldover design removes the confounding of two-factor interactions and main effects. Each column includes three zero values, which means that quadratic effects are estimable in each factor. And the center run in the last row enable you to include all main effects, quadratic effects for each factor, and an intercept term in the model.
 Run X1 X2 X3 X4 X5 X6 X7 X8 1 0 1 1 1 1 1 1 1 2 0 -1 -1 -1 -1 -1 -1 -1 3 1 0 1 1 -1 1 -1 -1 4 -1 0 -1 -1 1 -1 1 1 5 1 -1 0 1 1 -1 1 -1 6 -1 1 0 -1 -1 1 -1 1 7 1 -1 -1 0 1 1 -1 1 8 -1 1 1 0 -1 -1 1 -1 9 1 1 -1 -1 0 1 1 -1 10 -1 -1 1 1 0 -1 -1 1 11 1 -1 1 -1 -1 0 1 1 12 -1 1 -1 1 1 0 -1 -1 13 1 1 -1 1 -1 -1 0 1 14 -1 -1 1 -1 1 1 0 -1 15 1 1 1 -1 1 -1 -1 0 16 -1 -1 -1 1 -1 1 1 0 17 0 0 0 0 0 0 0 0