PIRLS and ePIRLS are sample-based assessments, which means that a sample of students are selected to take the assessments and their results used to generalize to a larger target population. PIRLS required all participating education systems to adhere to detailed technical standards (Martin, Mullis, and Hooper 2017), which provided guidance about the target population, sampling, response rates, translation, assessment administration, and data submission. All students enrolled in the grade that represents the fourth year of formal schooling constitutes the desired international target population, provided that their mean age at the time of testing was at least 9.5 years and that the grade level is calculated from the first year of the International Standard Classification of Education (ISCED) Level 1. (ISCED Level 1 is the equivalent of grade 1 in the United States.) For most education systems, the target grade is grade 4 or its national equivalent. All students enrolled in the target grade, regardless of their age, belong to the desired international target population. Additional detail can be found in the detailed technical notes, available online at https://timssandpirls.bc.edu/publications/pirls/2016-methods.html.
PIRLS guidelines call for a minimum of 150 schools to be sampled, with a minimum of 4,000 students assessed. The U.S. PIRLS 2016 national school sample consisted of 176 schools, which was higher than the international sampling minimum of 150 to offset anticipated school nonresponse and ineligibility. A total of 158 U.S. schools agreed to participate in PIRLS 2016, including 131 from the original sample and 27 sampled as replacements for nonparticipating schools from the original sample. Of the 158 U.S. schools that participated in PIRLS, 153 also participated in ePIRLS (ePIRLS was scheduled for the second of two days of testing; not all schools elected to participate). In total, 4,425 U.S. students participated in PIRLS and 4,090 of these students also participated in ePIRLS.
The overall weighted school response rate was 92 percent for PIRLS and 89 percent for ePIRLS. The overall weighted student response rate was 94 percent for PIRLS and 90 percent for ePIRLS, and the overall U.S. student exclusion rate was 4.8 percent for PIRLS and 4.9 percent for ePIRLS.