Observing behavior before & after a manipulation under multiple circumstances.
Used in certain situations, it may be unethical to remove a treatment in order to test its effect
Ex: instead of using 1 person to test drug. Use 3 people to see if yield same results. Gets rid of extraneous variables and flukes.
- P1 Drug Day: 10
- P2 Drug Day: 20
- P3 Drug Day: 30