In its most recent assessment of global learning standards, the OECD stated on Tuesday that the COVID school closures are only partially to blame for the dramatic decrease in the reading and math proficiency of teenagers in dozens of nations.

The Organization for Economic Cooperation and Development, based in Paris, reported that since it started testing the reading, arithmetic, and scientific proficiency of 15-year-olds every three years in 2000, it has witnessed some of the sharpest declines in performance.

Nearly 700,000 youths took the two-hour test last year in the OECD’s 38 mostly developed country members and 44-non members for the latest study, closely watched by policymakers as the largest international comparison of education performance.

Compared to when the tests were last conducted in 2018, reading performance fell by 10 points on average in OECD countries, and by 15 points in mathematics, a loss equivalent to three-quarters of a year’s worth of learning.

While more than half of the 81 countries surveyed saw declines, Germany, Iceland, the Netherlands, Norway and Poland saw particularly sharp drops in mathematics scores, the OECD said.

On average across the OECD, one out of four 15-year-olds tested as a low performer in maths, reading and science, which means they could not use basic algorithms or interpret simple texts, the study found.

“COVID probably played some role but I would not overrate it,” OECD director of education Andreas Schleicher told a news conference.