Issues in Randomization Outline

Block Randomization• Insure the # of patients assigned toeach treatment is not far out of balance– Block size = 6, 2 study interventions (Aand B)• AAABBB, BAABAB, ABABAB….• Variable block size (permuted)– An additional layer of blindness• Different distributions of a trait likegender in the two arms possibleStratified Randomization• A priori certain factors likelyimportant (e.g. Age, Gender)• Randomize so different levels ofthe factor are BALANCED betweentreatment groups• Cannot evaluate the stratificationvariableStratified Randomization• For each subgroup or strataperform a separate blockrandomization• Common strata– Clinical center, Age, Gender• Stratification MUST be taken intoaccount in the data analysis!6