Indicator D2. What is the student-teacher ratio and how big are classes?

By level of education

The ratio of students to teaching staff compares the number of students (full-time equivalents) to the number of teachers (full-time equivalents) at a given level of education and in similar types of institutions. This ratio does not take into account the amount of instruction time students have relative to the length of teachers’ working days, nor how much time teachers spend teaching. Therefore, it cannot be interpreted in terms of class sizes (Box D2.1).

On average across OECD countries, there are 15 students for every teacher at primary level but the ratio ranges from 9 to 1 in Greece and Luxembourg to 26 to 1 in Mexico. On average, there are fewer students per teacher at secondary level (13 students per teacher) than at primary level. This reduction in the student-teacher ratio between primary and secondary level may result from differences in annual instruction time (as instruction hours tend to increase with the education level, so does the number of teachers) or from differences in teaching hours (teaching time decreases with the level of education as teacher specialisation increases). Only Chile, Colombia, Costa Rica, Hungary, Mexico, and the Netherlands, have a larger student to teacher ratio at secondary level than at primary level (Table D2.1).

On average, the student-teacher ratio is about the same in lower and upper secondary education (13 students per teacher). In some countries, however, it varies widely between these two levels. This is the case in Finland, where there are over twice as many students per teacher at the upper secondary level than at the lower secondary level. In Mexico, it is the opposite case, with 33 students per teacher at lower secondary level compared to 22 at upper secondary level (Table D2.1).

At the upper secondary level, the programme orientation can strongly influence the student-teacher ratio although on average the ratios of students to teaching staff in upper secondary vocational and general programmes are similar (13 to 1). In about 40% of OECD countries with data, the ratio is greater in vocational programmes than in general ones. In Latvia, New Zealand and the United Kingdom, there are about 9 students more per teacher in vocational programmes than in general ones. In other countries such as Brazil and Mexico, the difference is reversed: there are over ten students more per teacher in general programmes (Figure D2.1).

A combination of several factors may influence the variation in student-teacher ratios between vocational and general upper secondary programmes. In some countries, vocational programmes are significantly work-based, so vocational students spend considerable time outside the school resulting in fewer teachers (OECD, 2017[5]). Countries where more than half of upper secondary vocational students are enrolled in combined school- and work-based programmes tend to have an equal or higher number of students per teacher in vocational than in general programmes. In contrast, in most countries where all upper secondary vocational students are enrolled in school-based programmes, the student-teacher ratio in general programmes tends to be the same or higher than in vocational ones.

However, programme type alone does not explain all differences between student-teacher ratio in vocational and general upper secondary education. Other factors, such as field of study, also influence the student-teacher ratio in vocational programmes. Some fields require greater instructor attention and supervision, particularly those where students have access to more sophisticated equipment (Hoeckel, 2008[6]). This may be particularly the case in technical fields such as engineering, manufacturing and construction, or some specialties in health and welfare. For example, Latvia and the United Kingdom have among the lowest shares of upper secondary vocational students graduating from the combined fields of engineering, manufacturing and construction and health and welfare across OECD countries (see Box B7.1). In both countries, there are nine more students per teacher in vocational than in general programmes, the highest difference across OECD countries. In contrast, the fields of study of upper secondary vocational graduates in Austria, Germany and Switzerland are more diversified, which may explain the similar student-teacher ratios across programme orientations in these countries. These differences have important implications for the cost of vocational instruction, as advanced vocational training in specialised fields of study requires both complex machinery and a greater level of human resources (Klein, 2001[7]). In most countries with available data, the cost per student in upper secondary vocational programmes is higher than in general ones (see Box C1.1).

At tertiary level, there are on average 15 students per teaching staff member. The student-teacher ratio ranges from 4 to 1 in Luxembourg to over 25 to 1 in Colombia and Indonesia. The difference in student-teaching staff ratios across short-cycle tertiary and bachelor’s, masters and doctoral or equivalent level varies across countries with available data. These results should be interpreted with caution, however, as the student-teacher ratio remains a limited measure of the level of teaching resources at tertiary level, where research staff may make up a significant share of academic personnel. Moreover, the relatively low enrolment in short-cycle tertiary in some countries limits comparability across tertiary levels (see Indicator B1).

Average class size in primary and lower secondary education

The indicator on class size is limited to primary and lower secondary education. Class sizes are difficult to define and compare at higher levels, as students are often split into several different classes at these levels, depending on the subject area.

At the primary level, the average class in OECD countries has 21 pupils. There are fewer than 25 pupils per class in all the countries with available data, with the exception of Chile, Israel, Japan, and the United Kingdom. At the lower secondary level, the average class in OECD countries has 23 students. Among all countries with available data, the number varies from fewer than 20 students per class in Estonia, Finland, Latvia, Lithuania, the Russian Federation and the Slovak Republic to 30 students or more per class in Chile, Colombia, Costa Rica and Japan (Table D2.3).

The number of students per class tends to increase between primary and lower secondary education, although to a varying extent across countries. In Costa Rica, this increase corresponds to 18 students, the highest difference across OECD countries. On the other hand, in the United Kingdom and, to a lesser extent, Australia, Chile, Finland, Hungary, Latvia and the Russian Federation, the number of students per class falls between these two levels of education (Table D2.3).

Class size in public and private institutions

Class size is one factor that parents may consider when deciding on a school for their children, particularly at lower levels of education. Hence, the difference in average class size between public and private schools (and between different types of private institutions) could influence enrolment in different types of institutions.

Differences in class sizes between public and private institutions are similar to those observed for student-teacher ratios. On average across OECD countries, class sizes do not differ between public and private institutions by more than one student per class in both primary and lower secondary education. However, in some countries (including Brazil, Colombia, the Czech Republic, Latvia, Poland, and the Russian Federation), the average class in public primary schools has more than six additional students compared to the average class in private schools (Table D2.3). However, with the exception of Brazil and Colombia, the private sector is relatively small in all of these countries, representing at most 6% of students at the primary level (OECD, 2019[13]). In contrast, in Chile, Greece and Korea, the average primary class in private institutions is larger than in public institutions by at least four students (Figure D2.3).

At the lower secondary level, where private institutions are more prevalent, the comparison of class size between public and private institutions shows even larger variations. For example, in Costa Rica, there are on average 15 students less per class in private institutions than in public ones. The average class in private lower secondary institutions is larger than in public institutions in 8 countries, smaller in 17 countries and the same in 7 countries (Figure D2.3).

Trends in average class size

Between 2005 and 2018, class sizes have fallen at primary level and lower secondary level on average across OECD countries. At primary level, while class size decreased by 2% on average, there are strong differences among countries. About half of countries experienced an increase between 2005 and 2018, with Mexico and the Russian Federation displaying the most notable increases of 21% and 29% respectively. In contrast, primary class sizes in Korea decreased by almost 30% over the same period, the highest decline over the same period across countries with data. At the lower secondary level, the average class size fell by 7% between 2005 and 2018. This average masks considerably larger changes in individual countries. In Greece and Korea, for example, the average class size in lower secondary education has decreased by about 20-25% over the past decade whereas it increased by 3% or more in France, and Spain (Table D2.3).

Changes in class size may reflect efforts to balance the supply of teachers with changes in the student population. Countries with lower than average class sizes may see an increasing trend. For example, small class sizes in Lithuania results from both a high concentration of teachers in the active population and a declining student base (Shewbridge et al., 2016[14]). As a result, primary class size has increased by 17% between 2005 and 2018 as new recruits to the teaching profession are limited. In 2018, only 5% of primary teachers were under the age of 30, one of the lowest across OECD countries. In contrast, countries such as Chile and Korea with higher than average primary class sizes have both seen a notable decline between 2005 and 2018 (Table D2.3). In Chile, this is likely due to a rise in primary teacher numbers, which increased by more than 10% since 2014. In contrast, in Korea, declining student numbers has likely contributed to the fall in class size (OECD, 2019[13]).

Class size, along with instruction time and teaching time, determines the number of teachers an education system requires. It is therefore an important parameter in understanding the level of expenditure allocated to staff compensation in education. Countries make different choices when investing their resources which affect the salary cost of teachers per student (Box D2.3).

Figure D2.3. Average class size, by level of education (2018)

Countries are ranked in descending order of the average class size at the primary level.

Source: OECD/UIS/Eurostat (2020), Table D2.3. See Source section for more information and Annex 3 for notes (https://doi.org/10.1787/69096873-en).

 StatLink https://doi.org/10.1787/888934165434

Box D2.3. Theoretical class size, one of the drivers of teachers’ salary cost

The salary cost of teachers per student is determined by four factors: teachers’ salaries, instruction time, teaching time and theoretical class sizes. The impact of the first factor is direct: higher salaries lead to higher salary costs. The other three factors affect the salary cost by changing the number of teachers needed, assuming that the number of students enrolled is constant. If instruction time increases or teaching time decreases, more teachers must be hired to keep class sizes constant. Similarly, more teachers must be hired in order to reduce class sizes while keeping everything else constant.

Figure D2.4. Contribution of various factors to salary cost of teachers per student in public institutions, primary education (2018)

USD converted using PPPs for private consumption

Notes: See Table D2.5, available on line, for notes on each factor.

1. Teachers' statutory salaries after 10 years of experience instead of 15 years.

2. Teachers' statutory salaries at the start of their career instead of after 15 years of experience.

Countries and economies are ranked in descending order of the difference between the salary cost of teachers per student and the OECD average.

Source: OECD (2020), Table D2.4, available on line. See Source section for more information and Annex 3 for notes (https://doi.org/10.1787/69096873-en).

 StatLink https://doi.org/10.1787/888934165453

When comparing a country’s salary cost to the OECD average, it is possible to determine the contribution of each of the four factors. For instance, it is possible to assess whether a given salary cost is above average because of higher salaries, higher instruction time, lower teaching time, smaller class sizes or a combination of these four factors. Changes to one of these factors may require compensating trade-offs among the other factors in order to keep the total salary cost constant.

Figure D2.4 shows the wide variety of combinations of the four factors across countries and their different effects on the salary cost of teachers. The size of the contribution of each factor compared to the OECD average depends on the difference between the factor itself and the relevant OECD average. The sum of each factor’s contribution equals the difference in salary cost between that country and the OECD average. For example, in Poland, the salary cost of primary teachers per student is USD 192 below the OECD average. Poland has smaller estimated class sizes (+ USD 975) and shorter teaching time (+ USD 953) than the OECD average, both of which push the salary cost of teachers up. However, this is more than compensated for by below-average teachers' salaries (- USD 1 357) and below-average instruction time (- USD 762), which push the cost down (Figure D2.4).

The analysis highlights different policies at primary level across countries with similar spending. For example, the teacher salary costs per student in Greece and Japan are very similar (about 3 000 USD), but how they allocate their resources differ substantially. Japan spends more than OECD countries on average on primary teachers’ salaries and primary teachers have slightly shorter teaching hours than on average across countries. It compensates for these costs with larger than average class sizes, and shorter students’ instruction time. In contrast, class sizes in Greece are significantly smaller than the average. Greece compensates for the higher cost of smaller classes with below-average teachers’ salaries at this level (Figure D2.4).