Central
Central composite design — full 2^k factorial core + 2k axial points + 1 centre point. Used to fit second-order response surface models (RSM). Requires every factor to be two-level (the factorial core); the substrate then adds 2k axial points at ± axialSpacing and one centre point replicated numCenterReps times.
axialSpacing is either the classical rotatable value (computed from k via CentralCompositeDesign.rotatableAxialSpacing) or an explicit user-supplied number.
The three replication knobs override the document-level ReplicationSpec entirely for CCDs — see this class's KDoc.
Fractional CCD cores (passing a fractional iterator to the substrate's CentralCompositeDesign primary constructor) are a substrate capability not exposed by this spec for v1 — see Phase E11 polish.