Clinical Trials

Clinical Trials: A Practical Guide ■❚❙❘Table 2. Example of block randomization using a block size of 4.Block Permutation Subject Treatment1 6 1 B2 B3 A4 A2 4 5 B6 A7 A8 B3 3 9 A10 B11 B12 A4 1 13 A14 A15 B16 B5 2 17 A18 B19 A20 B6 5 21 B22 A23 B24 Asubject, the number of subjects in each arm is equal. Therefore, each block mustcontain two patients on treatment A and two on treatment B.Step 1:Step 2:Step 3:Given a sample size of 24 and using a block size of 4, we need six blocks.There are six possible permutations that allow two As and two Bsin each box: AABB, ABAB, ABBA, BAAB, BABA and BBAA.The randomization code for blocks can be generated by producinga random-number list for permutations 1–6.Table 2 provides a listing of random permutations of A and B for each subjectusing this method. Note that after every four patients there is a balance of subjectsbetween treatments A and B. If we need more than six blocks (or have over24 patients), we can continue sampling more of the six possible block permutationsshown above. The procedure is repeated until all patients are randomized.69