The randomization code was developed using a computer random number generator to select random permuted blocks. Blocking Blocking can be used to ensure close balance of the numbers in each group at any time during the trial.
Improved balance comes at the cost of reducing the unpredictability of the sequence.
Blinding the interventions, larger block sizes, and randomly varying the block size can ameliorate this problem. Such imbalances can be avoided without sacrificing the advantages of randomization. Stratification ensures that the numbers of participants receiving each intervention are closely balanced within each stratum. Stratification by center is common in multicenter trials. Although the first patient is truly randomized, for each subsequent patient the treatment allocation is identified which would minimize the imbalance between groups at that time.