10,000 received extra milk, and 10,000 were controls. There was some randomization, but teachers were allowed to change assignment if they thought it unbalanced. Again, groups may not be comparable on all relevant predictors of outcome. Easily manipulated by someone who is unscrupulous or following a hunch. E.g., physician judgment of promising patients.
Groups are comparable, on average , on all predictors of outcome.
Some additional expense.