The phases of a clinical trial are Phase I, Phase II, Phase III, and sometimes Phase IV. Phase I focuses on safety and dosage. Phase II tests the effectiveness of the treatment. Phase III involves a larger group of patients and compares the treatment to existing treatments. Phase IV is post-marketing surveillance and is conducted after the treatment has been approved and marketed. It is designed to monitor the safety and effectiveness of the treatment in a larger population over a longer period of time. Each phase is designed to answer specific research questions and evaluate the safety and effectiveness of the treatment.