This section explains the indices derived from the PISA 2018 student, school, parent and ICT questionnaires used in this volume.

This annex discusses the PISA target population and the procedures used to select the sample that represented the target population. The information presented below is, for the most part, a summary of the information presented in of PISA 2018 Results (Volume I): What Students Know and Can Do (OECD, 2019[1]); the reader is invited to refer to that volume for more details. This annex also includes information specific to the financial literacy sample.

The statistics in this report represent estimates based on samples of students, rather than values that could be calculated if every student in every country had answered every question. Consequently, it is important to measure the degree of uncertainty of the estimates. In PISA, each estimate has an associated degree of uncertainty, which is expressed through a standard error. The use of confidence intervals provides a way to make inferences about the population parameters (e.g. means and proportions) in a manner that reflects the uncertainty associated with the sample estimates. If numerous different samples were drawn from the same population, according to the same procedures as the original sample, then in 95 out of 100 samples the calculated confidence interval would encompass the true population parameter. For many parameters, sample estimators follow a normal distribution and the 95% confidence interval can be constructed as the estimated parameter, plus or minus 1.96 times the associated standard error.

Quality assurance procedures were implemented in all parts of PISA 2018, as was done for all previous PISA surveys. The PISA 2018 Technical Standards (available on line at www.oecd.org/pisa/) specify the way in which PISA must be implemented in each country, economy and adjudicated region. International contractors monitor the implementation in each of these and adjudicate on their adherence to the standards.

When presenting the results by students’ gender, socio-economic status, education level and immigrant background, and schools’ socio-economic profile, location, type and concentration of immigrant students, the number of students and schools in each subsample has to meet the PISA reporting requirements of at least 30 students and 5 schools. Even when these reporting requirements are met, the reader should interpret the results cautiously when the number of students or schools is just above the reporting threshold. Tables III.A5.1 and III.A5.2, available on line, show the unweighted number of students and schools by student and school characteristics in the PISA 2018 sample so that the reader can interpret the results appropriately.