A researcher is interested in depression and the effectiveness of various treatments for depression. She decides to randomly assign each of 30 people seeking assistance with depression to one of three treatments -- behaviour modification, cognitive behavioural therapy, or rational-emotive therapy. Prior to treatment she interviews each person and rates the severity of the depression on a scale from one to ten. She repeats the interview and rating after 12 weeks of treatment. What are the relevant variables in this study? Which ones are independent and which are dependent? Are there variables that will have to be controlled in this study? What are they and how might they be controlled? Is this an example of cause and effect research, or is it normative research?