I'm Stick with a methodological problem and I'm not sure how to manage a possible solution. I want to compare two sets of observations made in two independent groups (15 subjects per group) over time. The two groups are primary cell cultures from clinical and a healthy control populations and the measured variable is the proliferation rate (continuous variable). Every time the culture plate is "filled", cells are resuspended and replated (the amount of cells at replating is the same for every subject in every group), and this is called a "passage" (a.k.a. the interval between cell plating for colture and cells filling the plate due to proliferation, with the need to resuspend and replate them). During specific passages (3,6,9,12) we collect some measurements of PR at specific time points (12,24,48,72 hrs), with 3 simultaneous determinations per subject. So we have, for every subject (and three times per time point): p3_12hrs, p3_24hrs, p3_48hrs, p6_12hrs, p6_24hrs, p6_48hrs and so on and so on.
I'd like to find a way to compare the two groups in terms of proliferation rate at the different passages, given the time dependency of the observations within the single passage and the putative time dependency between passages. Thanks in advance for your help!