[The different meanings of the word stratification]

In clinical research, stratification has three different meanings. In its first meaning, it describes the natural distribution of patients into subgroups, for instance patients may be stratified by stage. In its second meaning, stratification controls the random allocation of the treatments in a clinical trial, for instance the randomisation can be stratified on stage. Lastly, in its third meaning, stratification of a trial analysis consists into taking into account patients' characteristics in the analysis. This three meanings will be illustrated with a basic example.