Example of Create Response Surface Design (Central Composite)

A scientist wants to conduct an experiment to maximize crystal growth. Previous research has determined that catalyst exposure time, catalyst percentage, and temperature explain much of the variability in crystal growth.

The scientist generates the default central composite design for three factors and two blocks, assigns the factor levels, and randomizes the design.

Choose Stat > DOE > Response Surface > Create Response Surface Design.
Under Type of Design, select Central composite.
From Number of continuous factors, select 3.
Click Designs.
Select the second design (full design with 20 runs and 2 blocks) in the white box, and then click OK.
Click Results. Select Summary table and design table.
Click OK in each dialog box.

Interpret the results

The first table gives a summary of the design, which includes the total number of factors, runs, blocks, and replicates.

The design table displays the factor settings for each experimental run using coded factor names and levels. For example, in the first run, Factor A and Factor C are at level 1 and Factor B is at level −1. With 3 factors, the design has 20 runs. In the worksheet, Minitab displays the names of the factors and the uncoded levels.

Note

Minitab randomizes the design by default, so when you create this design, the run order will not match the order in the example output.

Design Summary

Factors:	3	Replicates:	1
Base runs:	20	Total runs:	20
Base blocks:	2	Total blocks:	2

α = 1.633

Two-level factorial: Full factorial

Point Types

Cube points:	8
Center points in cube:	4
Axial points:	6
Center points in axial:	2

Design Table (randomized)

Run	Blk	A	B	C
1	1	1.000	-1.000	1.000
2	1	-1.000	-1.000	-1.000
3	1	1.000	-1.000	-1.000
4	1	-1.000	-1.000	1.000
5	1	1.000	1.000	1.000
6	1	-1.000	1.000	1.000
7	1	-1.000	1.000	-1.000
8	1	0.000	0.000	0.000
9	1	1.000	1.000	-1.000
10	1	0.000	0.000	0.000
11	1	0.000	0.000	0.000
12	1	0.000	0.000	0.000
13	2	0.000	0.000	0.000
14	2	1.633	0.000	0.000
15	2	0.000	0.000	0.000
16	2	0.000	-1.633	0.000
17	2	0.000	0.000	-1.633
18	2	-1.633	0.000	0.000
19	2	0.000	1.633	0.000
20	2	0.000	0.000	1.633