Final answer:
The psychotherapy approach based on empirical data for treatment selection is evidence-based practice, and the researcher is likely conducting a randomized controlled trial. Freud's method of developing his personality theory through in-depth interviews with a few clients is known as a case study.
Step-by-step explanation:
The approach to psychotherapy that involves selecting the best treatment for a client based on empirical data is known as evidence-based practice. A researcher conducting a study using strict diagnostic criteria and randomly assigning participants to a treatment or control group is likely engaging in a randomized controlled trial (RCT). This type of study is essential for determining the efficacy of treatments as it compares outcomes between an experimental group receiving the treatment and a control group that does not. Utilizing this methodology allows researchers to control for variables and establish causation rather than mere correlation.
As for Sigmund Freud, he developed his theory of human personality through case studies. Unlike randomized controlled trials, case studies are in-depth investigations involving very few subjects and do not employ a control group for comparison. The placebo effect, experimenter bias, and random assignment are all critical considerations in experimental design to ensure that the observed outcomes are due to the treatment and not other factors.