Pharmaceutical Statistics Using SAS

9.3. Variations of Permuted Block Randomization

Although permuted block randomization provides a satisfactory allocation for the majority of clinical trials, some variations of it are also employed.

9.3.1. Permuted Block Design with a Variable Block Size

In a study unmasked to the investigator, permuted block design with block of variable size is sometimes employed (ICH E9), with the intention to make it harder for the investigator to guess the next treatment assignment and thus lessen the potential for selection bias. This strategy and its limitations are discussed in Rosenberger and Lachin (2002). They mention that, under a commonly used model for selection bias, variable block allocation "yields a substantial potential for selection bias ...