ICCS 2009 Technical Report - IEA

Stratification of schoolsPrior to sampling, schools were stratified. Strata are groups of units (schools in the case of ICCS)that share some common characteristic (e.g., geographic region, urbanization level, source offinancing). Generally, ICCS used stratification for the following reasons:• To improve the efficiency of the sample design—the national centers were asked toprovide stratification variables that were expected to be closely associated with students’learning-outcome variables;• To apply different sample designs, such as disproportionate sample allocations, to specificgroups of schools (e.g., states or provinces);• To ensure adequate representation of specific groups of interest (domains) of the targetpopulation in the sample.ICCS applied two different methods of stratification—one explicit, the other implicit.• If explicit strata were used, the total sample of schools was apportioned to the explicitstrata, and independent samples of schools were selected from each explicit stratum.• Implicit strata were used to sort or arrange schools within explicit strata.The combined use of implicit strata and systematic sampling is a way of ensuring a proportionalsample allocation of schools across all implicit strata. Each country applied differentstratification schemes after discussion with the IEA sampling team members. Appendix B of thisreport provides details about the stratification variables for each participant.School sampling frameIn order to prepare the selection of a sample of schools, the IEA sampling team asked nationalcenters to provide a list of schools with students enrolled in the target grade. (A comprehensivenational list of all eligible schools is called a school sampling frame.) The team carefully doublecheckedthe ICCS school-sampling frames in order to ensure that they provided completecoverage of the target population and did not include incorrect entries, duplicate entries, orentries that referred to elements that were not part of the target population. The team thenverified the plausibility of the information against official statistics.For each eligible school in the sampling frame, the sampling team required the followinginformation:• A unique identifier, such as a national identification number;• A measure of size (MOS) of the school, which was usually the number of students in thetarget grade or an adjacent grade;• Values for each of the intended stratification variables.School sample selectionIn order to select the school samples for the ICCS main survey, the sampling team used stratifiedPPS (probabilities proportional to size) systematic sampling. As noted earlier, this method iscustomary in most large-scale social surveys, and notably in most IEA surveys.The process of selecting the school samples from each country started with sorting the schoolsampling frame. The team sorted it by explicit strata, then within each explicit stratum byimplicit strata, and finally within each implicit stratum by MOS (alternately sorted in increasingand decreasing order).sampling design and implementation65