ICCS 2009 Technical Report - IEA

• The average class size in a country was so small that it was not possible to reach, throughselection of only 150 schools, the student sample size requirement of 3,000 students;• The NRC requested a sample-size increase in order to increase the amount of dataavailable for analysis.Because of non-participation, school closures, and inaccuracies in the school sampling frame,the achieved sample size of schools was smaller than the intended sample size in most of thecountries.In each sampled school, at least one classroom of the target grade was selected. In somecountries, more than one classroom was selected. This was done because:• The total number of schools in a country was so small that the student sample sizerequirements could not be met by selecting only one classroom per school;• The NRC had asked to select two classes to allow for class-level variance analysis;• Large sampling weight fluctuations would likely have otherwise occurred.Each country was required to have an achieved student sample size of 3,000 tested students.Because of non-response, school closures, decreasing student populations, or other reasons,some countries did not meet this requirement. The ICCS sampling team did not regard thisoutcome as problematic as long as the country met the overall participation rate requirements.Sample size in the teacher surveyThe school size requirements for the ICCS teacher survey were the same as those for thestudent survey. Within each selected school, a minimum intended teacher sample size of 15teachers was required. In schools with fewer than 15 teachers, all of the teachers were includedin the survey. If the number of eligible teachers was higher than 15, but fewer than or equalto 20, all teachers were selected to prevent a situation where only a few teachers were notincluded in the survey. ICCS did not specify a minimum achieved teacher sample size.Some NRCs requested all teachers in sampled schools who were teaching civic-related subjectsto be part of the national teacher sample. Other countries expressed the wish to select all homeclassteachers in a sampled school.The IEA Data Processing and Research Center (DPC) developed and provided the participatingcountries with specialized software called Windows ® Within-School Sampling Software (WinW3S)(IEA DPC, 2008). This gave countries the option of selecting defined groups of teachers withcertainty. In those countries that did choose this option, the overall number of teachers tosample in schools was systematically increased in order to prevent the remaining groups ofteachers from being under-represented in the sample.Overview of sample sizesTable 6.2 lists the intended and achieved school sample sizes, the achieved student sample sizes,and the achieved teacher sample sizes in the participating countries.School sampling designThe IEA DPC undertook the school sample selection for all of the ICCS countries. The DPCused as its general approach a stratified two-stage probability sampling design, in which theschools were selected systematically with probability proportional to size (PPS). The followingsubsections outline the school sample design for ICCS.sampling design and implementation63