Random assignment supports causal conclusions because it makes the groups:

1. A difference at the end only means something if the groups started alike. 2. With random assignment, all participants have an equal chance of being assigned to either group. 3. Unknown differences therefore spread evenly across the groups on average. 4. The groups are comparable rather than literally identical, which rules out the second option. 5. Assignment says nothing about group size or about what participants are told. _Source: OpenStax Psychology 2e (CC BY 4.0), Ch 2 'Psychological Research', sections 2.1-2.4_