DOE CCD
Dear community, I am working with 7 factors, and one of them is a quality factor. I was planning to start with a screening analysis using a fractional factorial design (2^7-2) resolution (IV). I would like to add some center points to test curvature, but one of my factors is not a number but is quality factor (like material), I am not pretty sure how to deal with this factor and how to process it...