The custom designer starts with a random set of points inside the range of each factor. The computational method is an iterative algorithm called coordinate exchange (Meyer and Nachtsheim, 1995). Each iteration of the algorithm involves testing every value of every factor in the design to determine if replacing that value increases the optimality criterion. If so, the new value replaces the old. This process continues until no replacement occurs for an entire iteration.
To avoid converging to a local optimum, the whole process is repeated several times using a different random start. The custom designer displays the best of these designs. For more details, see the section Optimality Criterion.
Sometimes a design problem can have several equivalent solutions. Equivalent solutions are designs with equal precision for estimating the model coefficients as a group. When this is true, the design algorithm may generate different (but equivalent) designs when you click the Back and Make Design buttons repeatedly.