Block randomization (also known as randomized block design) is a method in research design used to select and divide participants into different groups or conditions in order to avoid selection bias. (hyperlink?) It ensures that participants are assigned to conditions or groups with equal probability.
In research, randomization is essential to achieve balance in the effects (and possible risk factors in clinical trials) the participants are exposed to in each condition. Participant assignments are sequenced by using 'blocks'. Block randomization is a common method used to reduce the likelihood of bias and have the same number of participants for every condition group. This is especially useful if the sample size is small.