In fact the sampling requirements for international studies are very high and require independent external verification. There is simply no evidence this was a biased sample. It was representative of all schools in England in terms of national curriculum test results.
Moreover this research, for the first time in such a study, collected the national test results for the pupils in the sample. The proportions at level 4 and 5 are both within 1 per cent of the published figures for reading. This data is presented in its entirety in the PIRLS report which can be obtained from the National Foundation for Education Research or is downloadable from www.nfer.ac.uk Chris Whetton
National Foundation for Educational Research