Every three years, education ministers the world over keep a watchful eye on the publication of the Pisa rankings.

Run and compiled by the Organisation for Economic Cooperation and Development (OECD), the Programme for International Student Assessment (Pisa) tests a sample of 15-year-olds in each participating country on three areas: science, reading and mathematics.

In 2018, 79 countries took part, with over 600,000 students tested.

Read: Will Tuesday's Pisa results be Gove's report card?

Background: What is the Pisa test and what does it measure?

Analysis: England's heads 'the most data-driven'

Each country gets a report card: a score on each of the domains tested, which serves as the basis to create the Pisa rankings. The average of the scores on each subject becomes the OECD average.

How well has the UK done in Pisa education rankings?

But rankings, and any apparent drops down them, should be interpreted with caution. Firstly because the number of countries participating in Pisa varies from year to year so you may not be comparing like with like. Secondly because of concerns about the methodology behind the study.    

So, how does the UK fare?

Pisa 2018

• Scores in science continue a downward trend: 505 on average, down from 505 in 2015. However, the country ranked 14^th, one place higher than in 2015, probably because other countries have also achieved lower scores in science;
• In maths, a score of 502 signals a slight improvement from 2015: the UK is ranked 18^th, up nine places from the previous Pisa cycle;
• Reading, the focus of Pisa 2018, saw the UK achieve a score of 504. The country ranked 14^th, eight places higher than in 2015.

Pisa 2015

• In science, the UK achieved a score of 509, well above the OECD average of 493, and ranked 15th.
• In reading, its score was 498, which placed it in 22nd position, while the OECD average was 493.
• For maths, the UK ranked 27th, with 492 points, only just above the OECD average of 490.
   

A total of 72 countries participated in Pisa 2015.

Click here to access a graph showing the UK's performance in the Pisa 2015 rankings relative to the top 30 countries.

Pisa 2012

But how has the UK's position changed over time? Not substantially.

Three years before the 2015 rankings, 15-year-olds in the UK achieved broadly similar results:

• In science, the country got a score of 514, but finished 21st, six ranking positions below its 2015 results. The OECD average in science was 501.
• In reading, the UK average score was 499, above the OECD average of 496, leaving the country in 23rd position. 
• In maths, the UK had an average score of 494 (exactly in line with the OECD average) and took 26th position on the rankings.
   

In 2012, 65 countries participated in Pisa.