3. Modeling

Assigning the value " – 1" to the low level and "+ 1" to the high level of each factor amounts to making a change of unit of measurement and a change of origin. When a factor A varies from A ₁ to A ₂ , the origin is equal to A ₀ = 1/2 (A ₁ + A ₂ ). The transition from the variable "A" to the reduced centered variable "x" is made by the following formula:

where the pitch p = A ₂ – A ₀ ...