I need you assistance with finding the right model to analyze longitudinal data with a single group. My data comes from the ophthalmic field, i.e. eyes. I have n subjects. For each subject, one eye was chosen by random, and some treatment was applied to reduce the pressure inside the eye (a continuous scale).
The second eye was not treated, but the pressure was measured there as well. I have several time periods: Before treatment was applied, after 1 week, after 2 weeks and after a month.
I want to know if the treatment works, and basically could settle for a paired t-test using 1 time period and comparing it to the before values. However, I have noticed a correlation between the treated and untreated eyes. The correlation is 0.8!
So I thought that maybe, using the untreated eye, I could explain some of the variability. To sum it up, I have one group of treatment, I have 4 time periods and I have another eye for control. How do I analyze this for the best?