Hi I’m working with a physiologically based kinetic model where I’m comparing one compound to the other by dividing their area under the concentration-time curve (auc)

I code my codes using stan, but realize that for the output to be logical, I need to ensure that I’m dividing auc from individuals with the same physiology. For example, in one compound model, the auc I get for later division by the other auc from the second compound model should be from individual with low excretion (not low in one model, but high in the other model, as currently sampled with stan because it is random in a sense). Is there a way to fix the way stan sample from one parameter in two parallel models? Or random sampling is just the current state of the art?

Many thanks,

Frances