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…

Illustrates a method to divide large citation database files into clusters of journals without requiring the use of arbitrary starting points. Results are discussed relative to biomedical literature. (Author/FM)