I have had this issue come up a few times in doing a DOE where I have a set of units where each one can be reused in testing, but it is a specific set with specific settings that I want to model the impact of.
So, for example, I may have 12 units where 4 are from Category A, 3 are from Category B, and 5 are from Category C (shown in the table below). I am going to be testing with this category as a categorical factor, along with two continuous factors (temperature and humidity). But I can perform however many tests are needed. I'm interested in interaction effects and in second order effects. Is there a way to have a table set up with the units available and then re-use the units within the DOE platform? Whenever I try to use Add Factor >> Covariate, I am limited to at a max, the number of rows in the covariate table and cannot reuse units.