Final answer:
Observational study designs involve measuring characteristics in a sample without manipulating variables of interest. They are used in situations where experiments are impossible, impractical, or unethical. For example, researchers may use observational studies to investigate the effects of smoking on health.
Step-by-step explanation:
Observational study designs are a type of nonexperimental scientific investigation in which the researcher measures characteristics in a sample without manipulating variables of interest. There are three types of observational studies: cross-sectional, case-control, and cohort studies. These studies are used when experiments are impossible, impractical, or unethical to conduct. For example, researchers may use observational studies to investigate the effects of smoking on health, as it would be unethical to deliberately expose subjects to tobacco smoke.