PIRLS assesses the reading literacy of children in their fourth year of formal schooling. This student population was chosen for PIRLS because it is an important transition point in children’s development as readers. Typically, at this point, students have mastered the basics of learning to read and are now reading to learn. In many countries, this also is when students begin to have separate classes for different subjects, such as mathematics and science. 

PIRLS defines the fourth year of formal schooling according to the International Standard Classification of Education (ISCED) developed by the UNESCO Institute for Statistics1. The ISCED classification provides an international standard for describing levels of schooling across countries, and covers the full range of schooling, from early childhood education (Level 0) to doctoral study (Level 8). ISCED Level 1 corresponds to primary education, or the first stage of basic education, and is considered to be the first stage of formal schooling. 

The target population for PIRLS is defined as follows:

The PIRLS target grade should be the grade that represents four years of schooling, counting from the first year of ISCED Level 1.

The PIRLS target grade is four years after the beginning of Level 1, which is the fourth grade in most countries. However, given the linguistic and cognitive demands of reading, PIRLS wants to avoid assessing very young children. Thus, if the average age of fourth grade students at the time of testing would be less than 9.5 years, PIRLS recommends that countries assess the next higher grade (i.e., fifth grade).