The process of classifying patients, after they have been enrolled in the study, into defined subgroups (risk groups) on the basis of prognostic factors , as part of the randomization process or for purposes of data analysis.
A subset of the known prognostic factors (e.g., age, WBC count at diagnosis, presence of specific cytogenetic abnormalities) are used for the initial stratification of children with ALL into treatment groups with varying degrees of risk for treatment failure.