Pharmaceutical Statistics Using SAS

9.4. Allocations Balanced on Baseline Covariates

When certain subject characteristics (baseline covariates) are known to affect the response, it is often important to have the treatment groups balanced with respect to these covariates:

• A balanced allocation helps minimize bias due to covariate imbalances unaccounted for in the analysis model.

• A balance with respect to important covariates improves the efficiency of the analysis (Senn, 1997). Note, however, that in large trials the gain in power resulting from the use of a balanced allocation is typically small compared to post-stratified analysis without balanced allocation (McEntegart, 2003). At the same time, as pointed out by McEntegart, even a large trial can suffer from a substantial loss ...