This supplement contains documentation on the explicit and implicit stratification variables included in the PIRLS 2011 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. Details of the PIRLS 2011 sample design and implementation are available online at "Methods and Procedures in TIMSS and PIRLS 2011." [For the "PIRLS 2011 User Guide for the International Database," see ED544357. For Supplement 1, see ED544359. For Supplement 2, see ED544360. For Supplement 3, see ED544361.]