Just a month ago a report from the esteemed CEM unit at Durham University showed independent school pupils get close to a grade higher in every GCSE, and in 2015 exams 17% of independent pupils' A levels were graded A* compared to 6% in other schools.
It is clear that added value to age 16 is outstanding, but achieving extra value between GCSE and A level is difficult because independent school pupils come into that two-year period with very good GCSE grades. This effectively acts as a cap on value added, which, it's worth noting, is still higher than in non-independent schools.
If you look at the best indicator - actual A level grades - 84 of the top 100 schools recorded by official DfE data are independent schools. Of the 16 state schools in the top 100, most are very highly selective or grammar schools.
Over all year groups, independent schools educate around 7% of the UK's children, but this jumps to 12% for the 6th form years. It's therefore clear that parents who choose independent know they are getting something extra in the final two years of schooling.
The School Dash author interestingly notes that the 'discursive' humanities subjects thrive in independent schools as they require students to have the eloquence and self-confidence to make arguments on opinion rather than fact.
This helps crystallise the true value of independent schools for parents who pay for their child's education. While academic excellence remains a priority, independent schools offer so much more beyond the classroom, and these are factors which are difficult to measure in a graph.
Read more on the ISC website.