In addition to the following questions about TIMSS, more FAQs about international assessments are available at: http://nces.ed.gov/surveys/international/faqs.asp.
To provide valid estimates of student achievement and characteristics, TIMSS selects a sample of students that represents the full population of students in the fourth and eighth grades. This population was defined internationally as (1) all students enrolled in the grade corresponding to the fourth year of formal schooling, beginning with the first year of schooling as defined by the International Standard Classification of Education (ISCED), Level 1; and (2) all students enrolled in the grade corresponding to the eighth year of schooling, again beginning with ISCED Level 1.
The sample design employed by the TIMSS 2007 assessment is generally referred to as a three-stage stratified cluster sample. Schools were selected at the first stage with probability proportional to size (PPS), size being the estimated number of students enrolled in the target grade. The second-stage sampling units were classrooms within sampled schools. Countries were required to randomly select a minimum of one eligible classroom per target grade per school from a list of eligible classrooms prepared for each target grade. The third-stage sampling units were students within sampled classrooms. Generally, all students in a sampled classroom were to be selected for the assessment. However, it was possible to sample a subgroup of students within a classroom, but only after consultation with Statistics Canada, the organization serving as the sampling referee.
TIMSS guidelines call for a minimum of 150 schools to be sampled per grade, with a minimum of 4,000 students assessed per grade. The school response rate target was 85 percent for all countries. A minimum participation rate of 50 percent of schools from the original sample of schools was required for a country’s data to be included in the international database. The response rate target for classrooms was 95 percent, and the target student response rate was set at 85 percent, from both original and substitute schools.
U.S. sampling frame
The TIMSS U.S. sample is drawn from the Common Core of Data (CCD) listing of public schools supplemented with the Private School Universe Survey (PSS) listing of private schools. The combination of these national listings has proven to be close to 100 percent complete.
U.S. sampling design
The U.S. TIMSS sample used a three-stage stratified cluster sampling design. While the U.S. sampling frame was not explicitly stratified, it was implicitly stratified (that is, sorted for sampling) by four categorical stratification variables: type of school (public or private), region of the country (Northeast, Central, West, Southeast); community type (eight levels); and minority status (above or below 15 percent of the student population). The first stage made use of a systematic PPS technique to select schools for the original sample. The second stage consisted of selecting intact mathematics classes within each participating school. All students in sampled classrooms were selected for assessment. In this way, the overall sample design for the United States was intended to approximate a self-weighting sample of students as much as possible, with each fourth- or eighth-grade student having an equal probability of selection.
Countries are allowed to use substitute schools (selected during the sampling process) to increase the response rate once the 50 percent minimum participation rate of original school sampling is reached. In accordance with TIMSS guidelines, substitute schools are identified by assigning the two schools neighboring the sampled school in the frame as substitutes to be used in instances where an original sampled school refuses to participate. Substitute schools are required to be in the same implicit stratum (i.e., have similar demographic characteristics) as the sampled school.