Design Search Time is the amount of time allocated to finding an optimal design. Custom Design’s coordinate-exchange algorithm consists of finding near-optimal designs based on random starting designs. See Coordinate-Exchange Algorithm. The Design Search Time determines how many designs are constructed based on random starting designs.
You can specify your own value using the Design Search Time option. Increasing the search time tends to improve the optimality of the resulting design.
Keep in mind that designs produced by rerunning the algorithm can differ. Even with the same random seed, the numbers of starting designs used to construct the final design might differ because of variations in computing speed and other factors.
Note: The number of starting designs is given by the value in the Number of Starts text box. However, this value is not updated until after you construct your design by clicking Make Design.
In certain special cases, the globally D-optimal design is known from theory. These cases include:
• Two-level fractional factorial designs or nonregular orthogonal arrays. These are globally D-optimal for all main effect and two-factor interaction models.
• Latin-square designs. These are D-optimal for main effect models assuming the right sample size and numbers of levels of the factors.
• Plackett-Burman designs. These are D-optimal for main effect models.
If the coordinate-exchange algorithm detects that it has found an optimal design, it stops searching and returns that design.