Stratified randomization

Divide participants into strata defined by prespecified baseline prognostic variables and randomize independently within each stratum. Permuted blocks can keep treatment counts close throughout recruitment. This balances the stratification variable between arms by design, while allocation concealment keeps upcoming assignments hidden. Analyses should account for important design strata where appropriate.

Within each prespecified stratum, choose a block containing fixed equal counts of the two treatment assignments, then randomly permute their order. Every completed block gives exact treatment balance. Variable block sizes and allocation concealment protect against predicting the next assignment. Incomplete final blocks permit a small unavoidable imbalance; odd stratum sizes cannot have exact equal allocation.