State school pupils are lagging two years behind their counterparts in private schools by the age of 16, even when social background and prior attainment are taken into account, new research suggests.
The study – the first of its kind – found that on average independently educated pupils gain a premium of 0.64 of a grade on their GCSE results. Data from previous studies shows this is equivalent to two years of education.
At GCSE, independent schools have higher average scores in all subjects, the study showed, with the greatest differences between the sectors found in French, history and geography.
The smallest differences are found in chemistry, physics and biology.
If independent schools were measured on global Programme for International Student Assessment (Pisa) outcomes, the report said, they would outperform the best European nations and be level with Japan and South Korea.
The research – which compared performance throughout primary school and at GCSE – found that private school pupils were ahead at every level. Researchers from Durham University used data from the Performance Indicators in Primary Schools (PIPs) assessments and GCSE grades.
The findings come only three weeks after Ralph Lucas, editor-in-chief of The Good Schools Guide, told TES that improvements in state education were threatening to put independent schools out of business.
The new research from the Centre for Evaluation and Monitoring at Durham University attempted to control for differences such as prior ability, socioeconomic status and gender, in order to build up a picture of the genuine “academic value added” of private schools.
'Treat findings with caution'
However, the report’s authors said the techniques they employed – including the use of measures such as crude postcode-based deprivation data – meant that the results should be “viewed with caution”.
It was possible, the report said, that any causal effect of attending independent school might be an “overestimate” because of factors the study had not taken into account.
However, the report added: “Although there might be factors which we have not controlled for, the evidence from this study suggests that similar students achieve more in independent schools than in state schools when cross-sector differences are controlled [for pupil background and prior attainment].”
Julie Robinson, general secretary of the Independent Schools Council, which commissioned the research, said: "We're acutely aware of the difficulty in comparing different systems of schooling and drawing accurate conclusions, as there is much excellence to be seen in schools of all types.
“However, this ground-breaking report…really does give us solid ground to say that based on academic results, independent schools are worth paying for."
Barnaby Lenon, chair of the ISC, told TES that the findings illustrated how private schools were having an impact on pupils of all abilities, and their success was not just down to selection or privileged backgrounds.
Professor Robert Coe, one of the researchers at CEM who contributed to the study, said: “The availability of multiple assessments of children’s learning from large numbers of both independent and state schools over many years gives CEM a unique opportunity to compare the outcomes for children in the two sectors.
"It is always difficult to unpick the causes of any differences, and we think it is unlikely to be purely an effect of better teaching in independent schools, but we find a clear and significant difference in the GCSEs achieved that is not explained by any of the factors we can account for.”