In addition to the following questions about PIRLS, more FAQs about international assessments are available at: http://nces.ed.gov/surveys/international/faqs.asp.
Each participating country agrees to select a sample which is representative of the target population as a whole. In 2001, the target population was the upper of the two adjacent grades with the most 9-year-olds. For PIRLS 2006 and 2011, the definition of the target population has been refined to represent students in the grade that corresponds to four years of schooling, counting from the first year of International Standard Classification of Education (ISCED) Level 1—4th grade in most countries, including the United States. This population represents an important stage in the development of reading. At this point, generally children have learned to read and are using reading to learn. IEA's Trends in International Mathematics and Science Study (TIMSS) has also chosen to assess this target population of students.
In each administration of PIRLS, schools are randomly selected first (with a probability proportional to the estimated number of students enrolled in the target grade) and then one or two classrooms are randomly selected within each school. In 2001, a nationally representative sample of 3,763 U.S. 4th-grade students was selected from a sample of 174 schools. In 2006, a nationally representative sample of 5,190 U.S. 4th-grade students was selected from a sample of 183 schools. In 2011, a nationally representative sample of 12,726 U.S. 4th-grade students was selected from a sample of 370 schools.
The reason for a larger sample size in 2011 than in previous administrations of PIRLS was that in 2011 both TIMSS and PIRLS happened to coincide in the same year. The decision was made to draw a larger sample of schools and to request that both studies be administered in the same schools (where feasible), albeit to separate classroom samples of students. Thus, TIMSS (grade 4) and PIRLS in the United States were administered in the same schools but to separately sampled classrooms of students.