This supplement contains documentation on the explicit and implicit stratification variables included in the International Civic and Citizenship Education Study (ICCS) 2009 data files. The explicit strata are smaller sampling frames, created from the national sampling frames, from which national samples of schools were drawn. The implicit strata are nested within the explicit strata, and were used for sorting the sampling frame prior to the systematic sampling of the schools. For details of the ICCS 2009 sample design and implementation, please refer to the "ICCS 2009 Technical Report" (ED544622). [For the "ICCS 2009 User Guide for the International Database," see ED544615. For Supplement 2 (National Adaptations of International Questionnaires), see ED544616. For Supplement 3 (Variables Derived from the Survey Data), see ED544617. For Supplement 5 (ICCS 2009 Released Items), see ED544619.]