Randomization Works in Experiments, Even Without Balance

Randomization in experiments tends to balance covariates across test groups, but this balance is not guaranteed. Imbalances can arise from factors like small sample sizes, extreme data distributions, a large number of test groups, or simply bad luck in sampling. The core principle for experimental validity is not the perfect balance of covariates, but rather the independence between the treatment and these covariates, which is achieved through the randomization process itself. While balanced covariates are beneficial for achieving more precise estimates, their absence does not fundamentally undermine the validity of the experimental results.