ICCS 2009 Technical Report - IEA

National centers were able to define those groups of schools that had to be excluded in theirrespective national contexts from the ICCS student survey. Within-sample exclusions couldconsist of students with physical or mental disabilities or students who could not speak thelanguage of the test (typically, students with less than one year of instruction in the testlanguage). Any other types of within-sample student exclusions were not permitted. Detailsabout the exclusion categories for each country can be found in Appendix B of this report.Teacher exclusionsUnlike the situation regarding the student survey, there was no intention to exclude teachersfrom the ICCS survey. If a teacher was part of the teacher target population, he or she waseligible to participate in the study. Therefore, no minimum exclusion rates for teachers werespecified. However, teachers working at schools that were excluded did not have a chanceto participate, and thus had to be regarded as excluded. Each country was asked to provideinformation about the proportion of teachers in excluded schools. Because statistics aboutteachers per grade are rarely available, some countries could not provide exact figures, but onlyrough estimates, or no estimates at all.Overview of exclusionsTable 6.1 shows the population coverage and the exclusion rates for the student survey and theteacher survey for all ICCS countries.Sample size requirementsICCS set some limits on intended sample sizes (the number or expected number of selectedunits) and achieved sample sizes (the number of units that actually participated in the study) forboth the student and the teacher survey.Sample size in the student surveyThe overall goal of the student sample design was to achieve an effective sample size of at least400 students for the main variables of interest. This meant that the complex sample design ofICCS needed to yield the same sampling precision as a hypothetical simple random sample of400 students. Because students from the same schools tend to be more similar to one anotherthan students from different schools, it was necessary to survey a far larger number of studentsthan was needed to achieve this goal.The civic-knowledge score and questionnaire scales reflecting civic-related perceptions wereregarded as the main variables of interest. Given the international metric for these scales, theminimum requirements for sample precision were roughly equivalent to obtaining standarderrors for civic knowledge scores that did not exceed 5.0 score points and for questionnairescales that did not exceed 0.5 score points.The ICCS sampling team asked, with respect to the ICCS student survey, each participatingcountry to have a minimum intended school sample size of 150 selected schools. This meantselecting at least one intact class from each school. Once non-participation of schools andstudents had been taken into account, these requirements were expected to result in an achievedstudent sample size of roughly 3,000 tested students.Countries with fewer than 150 eligible schools included all schools in the survey. In severalcountries, more than 150 schools were selected. Increases in sample size could be implementedfor different reasons:• As shown in previous student surveys, variation in student achievement across schools in acountry can be large. This occurrence in the ICCS countries meant that the standards forsampling precision could only be met by increasing the school sample size;sampling design and implementation61