There are 31 countries where the mean reading score is at least 10 test points ahead of Wales, and 29 countries where the mean reading score is at least 10 test points lower.
Mae sgôr cymedrig 31 gwlad ar gyfer darllen o leiaf 10 pwynt prawf o flaen Cymru, ac mae sgôr cymedrig 29 gwlad ar gyfer darllen o leiaf 10 pwynt prawf yn is.