Stratified (subgroup) design

It is often used in diagnostic studies where some conditions or subgroups are rare, to make sure each stratum has enough subjects. The results then have to be weighted back to reflect the real population.

Design in which the target population is divided into subject subsets (or strata) and subjects are selected separately from each subset (or stratum).

FDA guidance: Design Considerations for Pivotal Clinical Investigations for Medical Devices (November 2013), Section 11, Glossary