At the graduate level, particularly for thesis work, a more focused topic may be appropriate. Original field research may be very specific. In cases where your specific topic has not been researched, your literature review will need to examine existing research that is related to your proposed study in some way.
If your target population group and method has not been studied, for example, your information sources may consider other therapies with the same population or the same therapy method with a different population group.
You will want to define parameters such as geographic range, ages of study participants, the chronological scope of your research, and other similar characteristics in order to keep your information sources as comparable as possible.