Central Composite Design (CCD)

The Central Composite Design (CCD) augments a two-level factorial core with axial "star points" at ±α and several centre points. It allows the estimation of a full quadratic model with main effects, two-factor interactions and quadratic terms — the typical basis of a response-surface analysis.

• Rotatable CCD (CCC) — star points at $\alpha =\sqrt[4]{2^k}$ outside the factorial cube; 5 levels per factor; rotatable (prediction variance direction-independent)
• Face-centered CCD (CCF) — star points on the cube faces ($\alpha =1$); 3 levels per factor; not rotatable but never violates hard factor limits
• Structure: $N=2^k+2k+n_c$ runs ($2^k$ corners + $2k$ star points + $n_c$ centre points)