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 It was sometimes the case that a sampled school was unable to participate in the assessment. In such cases, this originally sampled school needed to be replaced by replacement schools. The mechanism for selecting replacement schools, established a priori, identified the next school on the ordered school-sampling list as the replacement for each particular sampled school. The school after that was a second replacement, should it be necessary. Using either explicit or implicit stratification variables and ordering of the school sampling frame by size ensured that any original sampled school’s replacement would have similar characteristics. 3.3 Classroom and Student Sampling In the second sampling stage, classrooms of students were sampled. Generally, in each school, one classroom was sampled from each target grade, although some countries opted to sample two classrooms at the upper grade in order to be able to conduct special analyses. Most participants tested all students in selected classrooms, and in these instances the classrooms were selected with equal probabilities. The few participants who chose to subsample students within selected classrooms sampled classrooms with PPS. As an optional third sampling stage, participants with particularly large classrooms in their schools could decide to subsample a fixed number of students per selected classroom. This was done using a simple random sampling method whereby all students in a sampled classroom were assigned equal selection probabilities. 3.4 Performance Assessment Subsampling For the performance assessment, TIMSS participants were to sample at least 50 schools from those already selected for the written assessment, and from each school a sample of either 9 or 18 upper-grade students already selected for the written assessment. This yielded a sample of about 450 students in the upper grade of each population (eighth and fourth grades in most countries) in each country. For the performance assessment, in the interest of ensuring the quality of administration, countries could exclude additional schools if the schools had fewer than nine students in the upper grade or if the schools were in a remote region. The exclusion rate for the performance assessment sample was not to exceed 25% of the national desired population. 3.5 Response Rates Weighted and unweighted response rates were computed for each participating country by grade, at the school level, and at the student level. Overall response rates (combined school and student response rates) also were computed. 3 – 6 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 3.5.1 School-Level Response Rates The minimum acceptable school-level response rate, before the use of replacement schools, was set at 85%. This criterion was applied to the unweighted school-level response rate. School-level response rates were computed and reported by grade weighted and unweighted, with and without replacement schools. The general formula for computing weighted school-level response rates is shown in the following equation: R sch wgt ( ) = MOS / π MOS / π For each sampled school, the ratio of its measure of size (MOS) to its selection probability ( π i ) was computed. The weighted school-level response rate is the sum of the ratios for all participating schools divided by the sum of the ratios for all eligible schools. The unweighted school-level response rates are computed in a similar way, where all school ratios are set to one. This becomes simply the number of participating schools in the sample divided by the number of eligible schools in the sample. Since in most cases, in selecting the sample, the value of π i was set proportional to MOS i within each explicit stratum, it is generally the case that weighted and unweighted rates are similar. 3.5.2 Student-Level Response Rates Like the school-level response rate, the minimum acceptable student-level response rate was set at 85%. This criterion was applied to the unweighted student-level response rate. Student-level response rates were computed and reported by grade, weighted and unweighted. The general formula for computing student-level response rates is shown in the following equation: T I M S S D A T A B A S E U S E R G U I D E 3 – 7 ∑ part ∑ elig R ( stu)= wgt ∑ part ∑ elig 1/ p 1/ p where p j denotes the probability of selection of the student, incorporating all stages of selection. Thus the weighted student-level response rate is the sum of the inverse of the selection probabilities for all participating students divided by the sum of the inverse of the selection probabilities for all eligible students. The unweighted student response rates were computed in a similar way, but with each student contributing equal weight. i i i i j j
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Page 3 and 4: Contents Chapter 1: Overview of TIM
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