Central Composite Design (CCD) is one of the most popular designs used in RSM.
It is particularly useful when the goal is to fit a quadratic surface, which helps in understanding the curvature of the response surface.
CCD consists of three types of points:
1. Factorial Points:
These are the corners of a cube formed by the combinations of the factors at two levels (high and low).
2. Axial Points (Star Points):
These points are located along the axes at a distance α\alphaα from the center, where α is a value that can be chosen to ensure rotatability of the design.
3. Center Points:
These are points at the center of the experimental region.
They provide an estimate of the experimental error and check the curvature of the model.
A typical CCD includes:
where k is the number of factors.
Advantages of CCD:
It provides a good estimate of the second order (quadratic) effects.
It includes replication of the center points, which allows for a good estimate of experimental error.