Hi, I want to ask regarding the responses in FCCD RSM.

I have two factors considered in designing my sample. the responses set are the young modulus of elasticity, yield stress, and the derived of both responses such as the specific stiffness (young modulus/density) and specific strength (yield stress/density). Therefore, I have four responses.

typically in the compression test, the young modulus and yield stress show similar trends where structure with high young modulus usually has high yield stress, however, there are instances where the structure can have low young modulus but high yield stress (which is what I want for certain application where lower stiffness is desirable but must be able to sustain a certain load, this load-bearing capability is determined by the yield stress). meanwhile, the other two responses are added just for comparability for readers to others' study.

When I generate the contour plots for these four responses, the contour plots show a similar trend. however, in optimization, I was able to get 5 solutions to my desired criteria (low stiffness, high yield stress).

so my question is, are these responses appropriate to be used? because I was once told that the responses should be totally independent of each other.