I have an experiment with one independent variable with 4 levels (4 different treatments) and 1 dependent variable which was measured twice - before and after the treatment (repeated measures).
I think there are at least two ways to analyze this:
- Between-subject variable treatment (4 levels) with within-subject variable time (t1 & t2) and the dependent variable
- Between-subject variable treatment and delta values/change from time (t2-t1) as a dependent variable
What are the advantages and disadvantages of these methods? Which one has more power?