I'd like to make a custom design to develop a mixture with some optional additives. The basic design is:
Y (Performance)
X1 (Main component physical characteristic): Categorical - 2 level
X2 (Main component chemical characteristic): Categorical - 2 level
X3 (Additive type): Categorical - 2 level (A & B, only one or the other would be used, never in combination)
X4 (Additive concentration): Continuous or Discrete Numeric (0.1, 0.2, 0.3)
What is the best approach for making a design that will also evaluate performance when no additive is used?
- Can I simply add 0 as an option for X4? Or, will that weaken my ability to evaluate performance differences between additives (presumably Additive A and Additive B would both perform the same at 0)?
- Could I consider adding "none" as a level in X3, include 0 as the lower limit in X4, and use factor constraints to ensure that non-viable combinations (e.g., X3 = "none" & X4 = 10%, X3 = "A" & X4 = 0%) are excluded from the design?
- Is there a different approach that would be better?
Any guidance or suggestions would be appreciated.
