A researcher observed that people who have a large number of pets also live in houses with more bathrooms than people with fewer pets. Having a large number of bathrooms causes people to buy more pets. Some other variable may cause people to buy larger houses and to have more pets. Variables are investigated in more natural conditions. It is easier to hold extraneous variables constant.
Every test score is thought to have two components.
A food critic rates restaurants according to the quality of food, service, and atmosphere. She assigns four forks for excellent, three for good, two for fair, and one for poor. Dr. Bowman has developed a coding system for aggressive behaviors displayed in comic books. She has her assistants read comic books published between 1980 and 1984 and record the number of aggressive acts displayed in each comic. She also has her assistants read comic books published between 1985 and 1989 and record the number of aggressive acts displayed.