PISA 2025 Results (Volume I)

Annex A2. The PISA target population and the PISA samples

What is the PISA target population?

PISA assesses the cumulative outcomes of education and learning at a point at which most young people are still enrolled in formal education: when they are 15 years old. PISA uses an age-based definition of its target population, one that is not tied to the institutional structures of national education systems.1

The target population of PISA is defined as all students who are aged between 15 years and 3 (complete) months and 16 years and 2 (complete) months at the beginning of the assessment period,2 plus or minus an allowed 1-month variation, and who are enrolled in an educational institution at Grade 7 or higher. All students who met these criteria were eligible to sit the PISA assessment in 2025, regardless of the type of educational institution in which they were enrolled and whether they were enrolled in full- or part-time education. Table I.A2.1 presents the proportion of 15‑year‑old students enrolled in Grade 7 or higher, from PISA 2003 to 2025.

Hence, PISA measures knowledge, skills and attitudes of a group of individuals who were born within a comparable reference period, but who may have been exposed to different educational experiences inside and outside of school. These students may be distributed over different ranges of grade levels (both in terms of the specific grade levels and the spread in grade levels) in different countries/economies, or over different tracks or streams within their respective education systems. For example, Table I.A2.2 shows the percentage of students at each grade level, by country and economy. Table I.A2.3 presents the International Standard Classification of Education (ISCED) level in which students aged 15 are most frequently enrolled, by country and economy. It is important to consider these differences when comparing PISA results across countries and economies. In addition, differences in performance observed at age 15 are not static and may change over the life course.

How were students chosen?

In all PISA 2025 participating countries/economies, a two-stage stratified sample design was implemented. The first-stage sampling units consisted of individual schools having 15-year-old students, or with the possibility of having such students, at the time of assessment. Schools were sampled from a comprehensive national list of all PISA-eligible schools, known as the school sampling frame. The schools in the sampling frame were assigned to mutually exclusive groups (strata) based on school characteristics. Within strata, schools were sampled systematically with probabilities that were proportional to a measure of size. The measure of size was a function of the estimated number of PISA-eligible 15-year-old students enrolled in the school. This type of sampling is referred to as systematic with probability-proportional-to-size (PPS) sampling.

The second-stage sampling units were students within sampled schools. Once schools were selected to be in the sample, a complete list of each sampled school’s 15-year-old students was prepared. Countries/economies participating in the computer-based assessment (CBA) had to set a target cluster size (TCS) of 42 students, while countries/economies participating in the paper-based assessment (PBA) had to set a TCS of 35 students. Variations to the TCS were allowed, in consultation with the sampling contractors, for reasons such as expected student non-response or operational considerations, among others. From each list of eligible students within a school that contained more than the target cluster size, a sample of students were selected with equal probability; and for lists with fewer than the target number, all students on the list were selected. The students selected for the Foreign Language Assessment (FLA), which was optional, were an additional sample of students above and beyond those needed for the core PISA assessments.

Population coverage, and school and student participation rate standards

To provide valid estimates of student achievement, the sample of students had to be selected using established and professionally recognised principles of scientific probabilistic sampling in a way that ensured representation of the full target population of 15-year-old students in the participating countries and economies.

Furthermore, quality standards had to be maintained with respect to: i) coverage of the PISA international target population, ii) accuracy and precision, and iii) school and student response rates.

Chapter 14 of the PISA 2025 Technical Report (OECD, forthcoming[1]) provides sampling outcomes of the individual PISA‑participating countries and economies. PISA data and methodology can be found online in the PISA database. https://www.oecd.org/en/about/programmes/pisa/pisa-data.html

Coverage of the PISA international target population

It is often unavoidable to reduce coverage of the target population by excluding, for instance, a small, remote geographical region due to inaccessibility, language differences, political, organisational or operational reasons, or presence of special education needs students. Areas deemed to be part of a country/economy that included students in the PISA target population, but which were not included for sampling, are designated as non-covered areas. To ensure international comparability, efforts were made to ensure that exclusions, if necessary, were minimised according to the PISA 2025 Technical Standards (https://www.oecd.org/content/dam/oecd/en/about/programmes/edu/pisa/publications/technical-standards/PISA_2025_Technical_Standards.pdf).

Exclusion could also take place either at the school level (exclusion of entire schools) or at the within-school level (exclusion of individual students).3 These exclusions were often for special education needs or language differences. While the types of within-school exclusion are defined consistently across all participating countries and economies, a nationally-defined within-school exclusion category was permitted if agreed upon by the international contractor and the OECD.4 As in previous PISA assessments, the Une Heure (UH) form, which consists of a subset of the PISA assessment items deemed more suitable for students with special needs, was available for PISA 2025. Eighteen countries/economies used the UH booklet in PISA 2025.

A school attended only by students who would be excluded from taking the assessment for intellectual, functional or linguistic reasons was considered a school-level exclusion.

The overall exclusion rate within a country/economy (i.e. school-level and within-school exclusions combined) needed to be kept below 5% of the PISA desired target population. Not all countries and economies met this standard. See Chapter 14 of the PISA 2025 Technical Report (OECD, forthcoming[1]) for details on sampling outcomes of the individual PISA-participating countries and economies.

Accuracy and precision of the PISA samples

At minimum, 150 schools were selected in each country/economy, but if a participating country/economy had fewer than 150 schools for 15-year-old students in existence, the general practice was to select all schools for participation. Within each participating school, a predetermined number of students – the target cluster size (TCS), as defined earlier – was randomly selected with equal probability. In schools with fewer students than the target cluster size, all students were selected. In total, a minimum sample size of 6 300 assessed students was needed in computer-based countries/economies, or 5 250 assessed students in paper-based countries/economies. In countries/economies where the entire population had fewer students, all students were selected. It was possible to negotiate a target cluster size that differed from 42 students (or 35 for PBA countries). If the target cluster size was lower than 42 (or 35), the sample size of schools was increased to more than 150 to ensure that at least the minimum sample size of assessed students would be reached. The target cluster size selected per school had to be at least 25 students to ensure adequate accuracy in estimating variance components within and between schools.

Countries and economies participating in the FLA option needed an additional 1 650 assessed students for FLA.5 To accomplish this, the target cluster size was usually increased by approximately 26% for countries/economies participating in the FLA. For example, a country/economy that would have sampled 42 students in each school generally increased its TCS to 53 to accommodate the FLA sample. In some instances, the country/economy opted to increase the school sample size to achieve the required number of students selected for FLA.

Participating countries/economies were strongly encouraged to identify available variables to use for defining the explicit and implicit strata for schools to reduce the sampling variance. See further details in Chapter 7 of the PISA 2025 Technical Report (OECD, forthcoming[1]).

School response rates

At least 85% of the schools initially selected to take part in the PISA assessment were required to agree to conduct the test when accounting for the number of enrolled 15-year-olds (see details in the PISA 2025 Technical Standards, https://www.oecd.org/content/dam/oecd/en/about/programmes/edu/pisa/publications/technical-standards/PISA_2025_Technical_Standards.pdf). Where the initial response rate of schools was between 65% and 85%, however, an acceptable school response rate could still be achieved using replacement schools.

Whenever a school is selected for PISA, two other schools – the most similar according to the statistical criteria used for sampling – are selected as replacement schools in case of non-response or other contingencies. However, statistical similarities notwithstanding, sampling bias is still possible if the replacement schools differ from sampled schools in ways that might not be considered for sampling. Therefore, countries/economies were encouraged to persuade as many of the schools in the original sample as possible to participate.

Furthermore, a school with a student participation rate less than or equal to 33% was not considered a participating school, and data from such schools were not considered for analysis.

Student response rates

An overall response rate of 80% of selected students in participating schools was required. A student who had participated in the original or follow-up cognitive sessions was considered a participant if they responded to a minimum of five cognitive items or if they responded to at least one cognitive item and had completed selected questions from the background questionnaire. The overall student response rate was computed using only students from schools with a student response rate greater than 33%.6

Not all countries and economies met these school and student response rates standards. See Chapter 14 of the PISA 2025 Technical Report (OECD, forthcoming[1]) for details on sampling outcomes of the individual PISA-participating countries and economies.

Table I.A2.1. The PISA target population and the PISA samples – Tables for Annex A2

Table I.A2.1	Change in the enrolment of 15-year-olds in Grade 7 and above (PISA 2003 through PISA 2025)
Table I.A2.2	Modal ISCED level for 15-year-old students in the PISA 2025 sample
Table I.A2.3	Percentage of students at each grade level inthe PISA 2025 sample

Statlink

References

[1] OECD (forthcoming), PISA 2025 Technical Report, OECD Publishing, Paris.

Notes

← 1.  International education surveys must ensure comparable target populations across countries and economies. While assessing students at the same grade level can help, differences in education systems – such as entry age, early childhood provision and structure – make grades difficult to compare internationally.

← 2.  The international requirement for PISA 2025 was that the assessment had to be conducted during a 70‑day period, referred to as the testing period, between 1 March 2025 and 30 September 2025, unless otherwise agreed. Furthermore, testing was not permitted during the first six weeks of the school year due to a concern that student performance levels may be lower at the beginning of the academic year than at the end of the previous academic year, even after controlling for age.

← 3.  International within-school exclusion of students was allowed for the following groups:

• Intellectually disabled students: These are students who have a documented mental or emotional disability and who, in the professional opinion of qualified staff, are cognitively delayed such that they cannot be validly assessed in the PISA testing setting. This category includes students who are emotionally or mentally unable to follow even the general instructions of the test. Students could not be excluded solely because of poor academic performance or disciplinary problems.

• Functionally disabled students: These are students who are permanently physically disabled in such a way that they cannot be validly assessed in the PISA testing setting. However, functionally disabled students who could provide responses were included in the testing.

• Students with insufficient experience in the language of assessment: These are students who need to meet all of the following criteria: i) they are not native speakers of the assessment language(s), ii) they have limited proficiency in the assessment language(s), and iii) they have received less than one year of instruction in the assessment language(s).

• Students taught in a language of instruction for the main assessment domain for which no materials were available. PISA Technical Standard 1.12 notes that the PISA test is administered to a student in a language of instruction provided by the sampled school in the major domain of the test. Thus, if no test materials were available in the language in which the sampled student is taught, the student was excluded. For example, if a country/economy has testing materials in languages X, Y and Z, but a sampled student is taught in language A, then the student can be excluded since there are no testing materials available in the student’s language of instruction.

← 4.  A specific subgroup of students (e.g. students with severe dyslexia, dysgraphia or dyscalculia) could be identified for whom exclusion was necessary but for whom the first four within-school exclusion categories did not explicitly apply, so that a more specific within-school exclusion definition was needed.

← 5.  The desired target population for the FLA is a subpopulation of the PISA desired target population. Most countries/economies participating in the FLA confirmed that all, or nearly all, PISA-eligible students were also eligible for the FLA. For a few countries/economies, this was confirmed after the Field Trial. There were a small number of countries/economies for which a significant number of PISA-eligible students were not eligible for the FLA. For example, two countries/economies had a large proportion of students whose main language of instruction was English. For these few countries/economies, the target cluster size and school sample size were adjusted, as necessary, in order to achieve an expected yield of at least 1 650 FLA-assessed students.

← 6.  Weighted student response rates were used for assessing this standard. Each student was weighted by the reciprocal of their sample selection probability.