Final answer:
Observational studies are used when random assignment is unethical, allowing researchers to observe naturally occurring phenomena and draw relationships between variables. For example, an observational study conducted in 1950 found a strong association between smoking behavior and lung cancer.
Step-by-step explanation:
Observational studies do not randomly assign subjects to treatment and control groups. These studies are often used when it would be unethical to randomize subjects, such as comparing the mortality rate of smokers and non-smokers. Instead, researchers observe naturally occurring phenomena and sort the data into control and experimental conditions to draw relationships between the dependent and independent variables. For example, Doll and Hill conducted an observational study in 1950 on smoking and lung cancer, finding a strong association between past smoking behavior and current lung cancer status.