The National Curriculum of 1988 provided a common curriculum for all primary and secondary schools in the state school sector in England, Wales and Northern Ireland, and ensured that schools in all local authorities had a common curriculum. However, pupils with special educational needs had attainments that fell well below those that the National Curriculum was meant to measure. It was not until the publication of the P scales in 1998 that it became possible for schools in England to measure the attainments and progress of pupils whose attainment levels could not register on the National Curriculum scale. This study by Francis Ndaji and Peter Tymms, from the Centre for Evaluation and Monitoring at Durham University, examines a number of aspects of the validity of the P scales data. In many ways the results are reassuring. The teacher ratings discriminate the levels and show that each level is more difficult to attain than those below it. However, the subjects are not so well discriminated because they measure the same attributes. The P scales do not show any gender bias, indicating that teachers apply the level descriptions to both boys and girls in the same way. However, they seem to be applied in different ways to pupils with different categories of learning difficulties. The study also found that an attainment level in one subject area can be equated to the same level in another subject area. Despite suggestions from teachers, there is no evidence of a bottleneck at P8, or indeed, at any level of the P scales. In general, the P scales are working.