Assume, I have developed a new intervention for people with light depression. I want to compare the effectiveness of this intervention (E) with an existing intervention (C). For this, I recruit test subjects from the local psychotherapeutic ambulance and randomly assign them to either the experimental (E) or control group (C). The interesting dependent variable is of course depressivity.

Is it enough to compare the levels of depressivity post-treatment? Or do I need to measure depressivity pre-treatment as well? Why?