Simple randomization does not guarantee balance in the numbers in the study. In particular, if the patient's characteristics change over time (for example, patients are worse off early than after treatment), the imbalance cannot be corrected early. Block randomization is used to solve this problem. The main idea of randomization of patients is to divide the block into M blocks of size 2N, so that in each block N patients are assigned A and patients N are assigned to B. The block is then randomly selected. This method ensures equal treatment allocation per block provided the block is fully utilized.
For example: two treatments A, B and block size 2.2 = 4
Treatment allocation is possible within each block
AABB (2) BBAA, (3) ABAB, (4) BABA, (5) ABBA, (6) BAAB
The size of the block, depending on the number of treatments, should be short enough to prevent imbalance, and large enough to prevent guessing treatment allocation in each group during the study. The size of the block should be at least twice the number of treatment nodes. The size of the block is not stated in the study so that researchers are blind to it.
If the blocks are expressed, the treatment series in each block can be guessed. For example, in block 2N = 4, A A B must be B and in A A as B B can be deduced.
This can lead to (selection bias). The solution to prevent this error is to: (1) not reveal the block mechanism. (2). Use random block size.