User Guide for the TIMSS International Database.pdf - TIMSS and ...

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C H A P T E R 3 S A M P L I N G TIMSS used a grade-based definition of the target populations. In a few cases, TIMSS components were administered only to the upper grade of these populations (i.e., the performance assessment was conducted at the upper grade and some background questions were asked of the upper-grade students only). However, two adjacent grades were chosen to ensure extensive coverage of the same age cohort for most countries, thereby increasing the likelihood of producing useful agebased comparisons in addition to the grade-based analyses. The stated objective in TIMSS was that the effective population, the population actually sampled by TIMSS, be as close as possible to the International Desired Population. Figure 3.1 illustrates the relationship between the desired populations and the excluded populations at the country, school, and student levels. Figure 3. 1 Relationship Between the Desired Populations and Exclusions Effective Target Population National Desired Target Population National Defined Target Population International Desired Target Population Exclusions from National Coverage School-Level Exclusions Within-Sample Exclusions Using the International Desired Populations as a basis, participating countries had to operationally define their populations for sampling purposes. Some NRCs had to restrict coverage at the country level, for example, by excluding remote regions or a segment of the educational system. Thus, the Nationally Desired Population sometimes differed from the International Desired Population. The basic sample design used by TIMSS is generally referred to as a two-stage stratified cluster sample design. The first stage consisted of a sample of schools, which may be stratified; the second stage consisted of samples of classrooms from each eligible target grade in sampled schools. In some countries, a third stage consisted of sampling students within classrooms. Exclusions could occur at the school level, student level, or both. TIMSS participants were expected to keep such exclusions to no more than 10% of the national desired populations. Nevertheless, the National Defined Populations generally were subsets of the desired populations. Participants could exclude schools from the sampling frame if they were in geographically remote regions, were extremely small, offered curriculum or structure different from the mainstream, or provided instruction only to students in the “within-school” exclusion categories. The general TIMSS rules for defining within-school exclusions follow. • Educable mentally disabled students. These are students who are considered, in the professional opinion of the school principal or other qualified staff members, to be educable mentally disabled students, or who have been so diagnosed in psychological 3 – 4 T I M S S D A T A B A S E U S E R G U I D E
S A M P L I N G C H A P T E R 3 tests. This includes students who are emotionally or mentally unable to follow even the general instructions of the TIMSS test. It does not include students who merely exhibit poor academic performance or discipline problems. • Functionally disabled students. These are students who are permanently physically disabled in such a way that they could not perform in the TIMSS tests. Functionally disabled students who could perform in the TIMSS test were included in the testing. • Non-native-language speakers. These are students who cannot read or speak the language of the test and so could not overcome the language barrier of testing. Typically, students who had received less than one year of instruction in the language of the test were excluded, but this definition was adapted in different countries. Some countries opted to test students in more than one language. 3.2 School Sample Selection In the first stage of sampling, representative samples of schools were selected from sampling frames (comprehensive lists of all eligible students). The TIMSS standard for sampling precision required that all population samples have an effective sample size of at least 400 students for the main criterion variables. To meet the standard, at least 150 schools were to be selected per target population. However, the clustering effect of sampling classrooms rather than students was also considered in determining the overall sample size for TIMSS. Because the magnitude of the clustering effect is determined by the size of the cluster and the intraclass correlation, TIMSS produced sample-design tables showing the number of schools to sample for a range of intraclass correlations and minimum-cluster-size values. Some countries needed to sample more than 150 schools. Countries, however, were asked to sample 150 schools even if the estimated number of schools to sample fell below 150. Information about design effect and effective sample size can be found in Gonzalez and Foy (1997). The sample-selection method used for first-stage sampling was based on a systematic probabilityproportional-to-size (PPS) technique. The schools in each explicit stratum (e.g., geographical region, public/private, etc.) were listed in order of the implicit stratification variables, and then further sorted according to their measure of size (MOS). Of course, the stratification variables differed from country to country. Small schools were handled either through explicit stratification or through the use of pseudo-schools. In some very large countries, there was a preliminary sampling stage before schools were sampled, in which the country was divided into primary sampling units. T I M S S D A T A B A S E U S E R G U I D E 3 – 5
Page 1 and 2: International Association for the E
Page 3 and 4: Contents Chapter 1: Overview of TIM
Page 5 and 6: C O N T E N T S 7.3 Codebook Files
Page 7 and 8: C O N T E N T S Table 7.7 Classific
Page 9 and 10: Chapter 1 Overview of TIMSS and the
Page 11 and 12: I N T R O D U C T I O N C H A P T E
Page 17 and 18: Chapter 2 TIMSS Instruments and Boo
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C H A P T E R 9 P E R F O R M I N G
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R E F E R E N C E S Gonzalez, E.J.
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R E F E R E N C E S Wilson, M.R. (1
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A C K N O W L E D G M E N T S Inter
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A C K N O W L E D M E N T S TIMSS A
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