Randomization

Summary

Randomization is the assignment of treatments to experimental units by a chance mechanism with known probabilities. It balances, on average, all unit characteristics between the treatment groups, including those that nobody measured. So differences in outcome can be attributed to the treatments. It also provides the probability model behind randomization tests and, in large samples, justifies the usual tests of analysis of variance. It is one of the three principles of design of experiments and the basis of randomized controlled trials and of causal inference.