Among the problems of devising national tables showing the extent to which schools do well by pupils with differing abilities is keeping tabs on children when they change schools.
According to David Hawker, assistant chief executive for assessment at the School Curriculum and Assessment Authority, the problems in producing value-added tables for 1998 include taking account of the high turnover of pupils in some schools, particularly primaries, and producing valid results when results are based on small number of pupils.
However, the academics advising SCAA are also suggesting that a value-added table, a measure of school effectiveness, cannot be based at secondary level on the current benchmark of five higher-grade GCSEs.
The results of the two-year project carried out for SCAA by Professor Carol Fitz-Gibbon and Peter Tymms, of Durham University, suggest that in order to be useful the results would have to be given across subject areas; the value-added tables would have to rank schools on the basis of results in science, mathematics, humanities and modern languages.
The problem for ministers is that the ranking of schools by subject results would not provide any direct comparison with the present tables based on the actual results across the range of GCSEs.
In addition, the project team want ministers to accept that schools should be allowed not to have to include in their results up to 10 per cent of pupils.
Professor Fitz-gibbon said if schools had to count every pupil they might be reluctant to take those who migrated between schools or who were non-English speakers.
Professor Fitz-Gibbon is also concerned that schools may be required to set targets for improvement on the basis of only one year's value-added results.
"Schools need at least three years' results on which to base any targets for improvement," she says.
However, Professor Fitz-Gibbon believes the proposals being put by SCAA will mean that the country will be the first in the world to have devised a system for measuring the added value across all ages in schools. "We will be unique in the world. This is a revolutionary step forward," she says.