 Most of these designs can be split into groups (blocks), for each of which the model will have a different constant term, in such a way that the block constants will be uncorrelated with the other coefficients.These designs can be augmented with positive and negative "axial points", as in central composite designs, but, in this case, to estimate univariate cubic and quartic effects, with length α = min(2, (int(1.5 + "K"/4))1/2), for "K" factors, roughly to approximate original design points' distances from the centre.Plackett–Burman designs can be used to construct smaller or larger Box–Behnkens, in which case, axial points of length "α" = (("K" + 1)/2)1/2 better approximate original design points' distances from the centre.