Every three years there’s a part of the test that asks the same questions as were previously asked to improve the test-on-test results.
The methodology has been used since 2000, in the case of reading, making it possible to draw long-term conclusions about comparable performance. Maths was introduced in 2003 and science in 2006.
PISA is intended to assess young people’s ability to apply their knowledge and skills to real-life situations. The word “literacy” is used to describe the results in maths, science and reading because researchers want to describe students big-picture abilities, not just their ability to literally read or add up.