Random group assignment allocates people or units to treatment options by chance rather than by a characteristic such as usage frequency. This prevents assignment rules from confounding treatment effects with preexisting differences. section 1.3.2 section 1.8
A website A/B test
Suppose a website compares versions A and B. Assign each user through a virtual coin toss tied to a persistent user ID, so the user keeps the same version across visits. With enough users, visit frequency, age, location, and other influential characteristics should be distributed similarly between the groups. section 1.3.2
Random assignment helps because it:
- Tests treatment options under the same conditions. section 1.3.2
- Reduces doubt that an outcome difference reflects selected participants rather than the treatment. section 1.3.2
- Provides the clearest estimates of causal effects when an experiment is feasible. section 1.8

