Preprimary school pupil-teacher ratio is the average number of pupils per teacher in preprimary school. Development relevance: The pupil-teacher ratio is often used to compare the quality of schooling across countries, but it is often weakly related to student learning and quality of education. Limitations and exceptions: The comparability of pupil-teacher ratios across countries is affected by the definition of teachers and by differences in class size by grade and in the number of hours taught, as well as the different practices countries employ such as part-time teachers, school shifts, and multi-grade classes. Moreover, the underlying enrollment levels are subject to a variety of reporting errors. Statistical concept and methodology: Pupil-teacher ratio is calculated by dividing the number of students at the specified level of education by the number of teachers at the same level of education. Data on education are collected by the UNESCO Institute for Statistics from official responses to its annual education survey. All the data are mapped to the International Standard Classification of Education (ISCED) to ensure the comparability of education programs at the international level. The current version was formally adopted by UNESCO Member States in 2011. The reference years reflect the school year for which the data are presented. In some countries the school year spans two calendar years (for example, from September 2010 to June 2011); in these cases the reference year refers to the year in which the school year ended (2011 in the example).