In the PISA rankings, done every three years by the Organization for Economic Co-operation and Development (OECD) to produce comparable data on education policy and outcomes across countries, Vietnamese students ranked 13th out of 79 countries and territories in reading comprehension, up 19 ranks from 2015.

They ranked 24th in mathematics, down two places, and fourth in science, up eight places.

But Vietnam was not included in the 2018 rankings since its data showed large discrepancies compared with the OECD’s model for evaluation and that the OECD wanted to study its results more, Le Thi My Ha, PISA’s Vietnam national director and director of the Ministry of Education and Training’s education quality assessment center, said.

The discrepancies arose because Vietnam's tests were paper-based while other countries' were computer-based, she said.

But the OECD did conclude that Vietnam carried out its testing in accordance with proper procedures through data analysis, personnel surveys and other methods, and there was no sign it had manipulated the results or showed bias in the grading process, she said.

The global rankings had nearly been completed by September, and so there was not enough time to include Vietnam's results in the list, she explained.

The PISA tests can be taken either on paper or on computers.

While both have similar questions, the actual testing procedures are vastly different, and the OECD needs to analyze and compare the results of the two, she added.

Paper-based testing was done in nine countries and territories, Argentina, Jordan, Lebanon, Moldova, North Macedonia, Romania, Saudi Arabia, Ukraine, and Vietnam.

"All countries which tested on paper had different results compared to those that tested on computers."

She said Vietnam's results were "significantly higher" than the others that also tested on paper.

Vietnam is considering switching to computer-based tests in future, she added.

PISA assesses 15-year-old students in science, mathematics and reading comprehension.

Vietnamese students first took the tests in 2012.