Create Response Surface Design - Central Composite Design
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Stat > DOE > Response Surface < Create Response Surface Design > choose Central composite > Designs

Generates central composite designs for 2 to 10 factors. You can also add center points and specify a value for a.

Dialog box items

The list box at the top of the Design subdialog box shows all available designs for the number of factors you selected in the main Create Response Surface Design dialog box. Highlight your design choice. The design you choose will affect the possible dialog and subdialog box options.

Number of Center Points Adds the specified number of center points to the design. If the design is blocked the center points are divided equally among the blocks. If the number specified is not a multiple of the number of blocks, then each of the last few blocks will have one less point than the other blocks.

Default: Choose to have Minitab determine the number of center points based on the design you specified. See the text box at the top of this dialog box for the default number of center points.

Custom: Choose to specify the number of center points you want to add to your base design (before replicates are added to the design).

Cube block: Enter the number of center points for the cube blocks.

Axial block:  Enter the number of center points for the axial block.

Tip

Use <Tab> to move between the boxes.

Value of Alpha

Default: Choose to have Minitab determine the value of alpha (a) based on the design you selected. See the text box at the top of this dialog box for the default value. The default values of alpha provide orthogonal blocking and, whenever possible, rotatability.

Face centered: Choose to generate a face-centered design (a = 1).

Custom: Choose to specify the a value; then enter the desired value. A value less than 1 places the axial points inside the cube; a value greater than 1 places them outside the cube.

Note

A value of a = (F)**1/4, where F is the number of factorial points in the design, guarantees rotatability.

Number of replicates: Enter the number of replicates up to 50.

Block on replicates: Check to block the design on replicates. If your design has no existing blocks, Minitab places each set of replicates in a separate block. If your design already includes blocks, Minitab replicates the existing blocking scheme, placing replicates in new blocks. For more information, see Blocking a Central Composite Design.