Treatment allocation ratio is the ratio of the number of subjects allocated to the two treatments in a clinical trial. The equal allocation is most usual in practice, but it might be advisable to allocate patients randomly in the other ratios when comparing a new treatment with the old one, or when one treatment is quite difficult or expensive to administer. The chance of detecting the real difference between the two treatments is not reduced much as long as the m ratio is not more extreme than 2:1.