In comparing two treatments, eligible subjects come to the experiment sequentially and must be treated at once. To reduce experimental bias and to increase the precision of inference about treatment effects, the adaptive biased coin design, which offers a compromise between perfect balance and complete randomization, is proposed and analyzed. This new design has the property that it forces a small-sized experiment to be balanced, but tends toward the complete randomization scheme as the size of the experiment increases.
"The Adaptive Biased Coin Design for Sequential Experiments." Ann. Statist. 6 (1) 92 - 100, January, 1978. https://doi.org/10.1214/aos/1176344068