Epidemiological studies makes use of both experimental and observational studies. Experimental studies are ones where the epidemiologist can control and manipulate different variables throughout the experiment, and usually involves a placebo treatment/group. This type of study is used when epidemiologists are trying to determine the cause of a health issue/disease or evaluating the effectiveness of a cure or interventions. Observational studies include descriptive and analytical studies; descriptive studies investigates epidemiological cases with regards to TPP while analytical study investigates hypothesis regarding relationships between health issues and risk factors. While descriptive studies (TPP) answers questions of when, who, and where, analytical studies tries to answer the question of how a population is affected by a disease and why they are affected. Overall, observational studies do not manipulate any variables and often uses comparison groups for analysis; this type of study is often done in an attempt to discover the links between exposure to certain risk factors and health outcomes. Some examples of observational studies include cohort studies, case-control studies, and cross-sectional studies.