# How to compare two groups with respect to the relative change in outcomes after treatment when there are two repeated measurements — pre/post test?

There are two groups of patients, say, young and old, and we want to determine whether these two groups of patients react differently to our treatment. Let $$X$$ be a continuous dependent variable, say, blood pressure which will be measured twice for every patient: once before the treatment and once after. An average change of $$X$$ after the treatment will be calculated for the first and second group, say, values $$A_1$$ and $$A_2$$, respectively.

1. How can we check whether there is a statistically significant difference between $$A_1$$ and $$A_2$$?
2. How can we conduct power analysis to estimate the size of the groups if we know only the measurement error of $$X$$ and the expected difference between $$A_1$$ and $$A_2$$?
3. Questions 1 and 2 is for the scenario when we measure not absolute, but relative changes in variable $$X$$.
• Is A1 and A2 different treatments? If so, that would mean both groups are “treated” in the second time period? – Thomas Bilach Mar 30 at 6:31
• Thank you for your question! I just edited my question for clarity. – Mark Mar 30 at 16:09