I am trying to run a sensitivity analysis on an efficiency indicator: the ratio between nitrogen outputs (milk, meat, crops) and nitrogen inputs (fertilizers, cattle feed, symbiotic fixation…).
I have 4 output variables and 12 input variables. However, they are not independent. For instance, fertilizer input is correlated with crop output. It seems that the methods I read about (ANOVA, Morris, Sobol) are based on the hypothesis that all variables are independent. This allows testing the effect of the variation of each variable while keeping all other variables constant.
Do you have any suggestion of how I could take these correlations into account in my sensitivity analysis ?