A. Descriptive in nature; useful in the early stages of research about a particular disease.
B. Participants are randomly allocated to a group and the intervention is delivered under tightly controlled conditions to avoid systematic errors.
C. Observational studies of people with common characteristics or experiences which are used to determine the prognosis and progress of disease over time.
D. Used when random assignment to the experimental and control group is not possible or is unethical.
E. Used to determine the frequency of disease, events, behaviors, attitudes, and other data at one point in time.
F. Studies where people with a particular diagnosis or disease of interest are matched with people who do not have the disease; data from the two groups are compared to explore what differences exist between the groups and to identify characteristics that may contribute to the disease.