Here was a definite bias - against the vaccine. Many school districts saw these flaws in the NFIP design, and used a different design. Otherwise, the effect of family background would have been confounded with the effect of the vaccine. The next issue was how to assign the children to treatment to control. Human judgment seems to be needed, to make the control group like the treatment group with respect to the relevant variables - family income, or the children's general health, personality, and social habits.
Experience shows, however, that human judgments often result in substantial bias.
It is better to use a carefully designed chance procedure. For the Salk trial, the procedure was equivalent to tossing a coin for each child, with a 50-50 chance of assignment to the treatment or the control group. Such a procedure is objective and impartial. And the laws of chance guarantee that with enough subjects, the treatment group and the control group will resemble each other very closely with respect to all the important variables, whether or not these have been identified. When an impartial chance procedure is used to assign the subjects to treatment or control, the experiment is said to be randomized controlled .