However, I should like to point out that the two samples did not have equal amounts of schooling. The children in Scotland were tested in Primary 5 (mean age 9.9 years) and the children in England were tested in Year 5 (mean age 10.3 years). The children in England will have spent between one and three terms in reception class before starting Year 1, so will have been at school longer.
Furthermore, I have found that the reading curriculum in reception in England is very similar to that of Primary 1 in Scotland. These differences in time at school may have affected the measured levels of reading attainment.
International surveys are very interesting but need to be taken with a pinch of salt as they do not have the tight control of variables that an experimental study would have.
Professor Rhona S Johnston
Department of Psychology, University of Hull.