In this study, the relevant variables are the severity of the depression (the dependent variable) and the treatment type (independent variable).
The study is concerned with the effectiveness of the different treatments on depression. Certain variables will have to be controlled. One would be the conditions under which the treatment is provided, with the differences between the treatments minimized to an appropriate extent. So for instance, therapist experience should be controlled, with the therapists providing each treatment ideally having comparable levels of experience. Otherwise, the differences in treatment effects may not have to do with the treatment itself, but with the superior experience and competence of the therapists used for one particular treatment modality. This could be controlled by ensuring the therapists have comparable levels of experience, and the therapies and administered in contexts that are similar as possible while staying true to the methodology of each therapy . This is an example of causal research because it is concerned with manipulating the independent variable (treatment type) to isolate it as the cause behind the effect of depression change.