Random Assignment
1. assign Ps to treatment and control groups such that all Ps have the same probability of being placed in a given group
2. Distributes impact of uncontrolled variables evenly across all groups
3. what about matching "matched groups", matching on; known correlates, purposeful matching to both conditions
4. note- not the same as random sampling (taking a sample from a population in a random way to get a representative of a population)