How do I compare independent schools to state schools?

Difficult. In one way a good private school does offer a more rounded education than most state schools, because they have the resources to provide a much richer cultural environment, a broader range of subjects and more extra curricular opportunities

Obviously not “rounder” in terms of mixing with a diverse range of people in socio/economic terms.

I'm laughing uproariously at the belief that private education offers 'a more rounded education and preparation for life'. How does cutting your child off into an elite sector do that?! And different nationalities are going to be the norm in pretty much any state school in a city and they'll be far more diverse than in the private sector..

Is your only criteria for comparison what exam grades students get? Because if so your missing a large part of the point of independent schools.

Without double checking, Marlboroughs exam results arent great yet it is considered one of the best independents in the country (not by me)

IMO its about exposure to a more rounded education and preparation for life, its about opportunities for different subjects, more sports, being amongst different nationalities (our school anyway), different thinking, confidence.....

I dont think all schools suit all children whether private or state, you need to know what suits your own child, not reduce the decision to how many random children got 5 A*s last year.

Assume that your child will get broadly similar exam results wherever they go. Because they will. And anyway, it is pretty near impossible to compare state/private results because the cohorts are so different.

Look at all the other stuff. It is difficult to match the cultural capital your money buys in a good private school ( I emphasize good because some are rubbish) because there just aren’t the hours in the day. Go and look. Ask loads of questions. Think about what you want to spend your money on. Think about whether private education matches your political/philosophical values. Think about where your child would be happy.

IMO the only sensible test is to look at the percentage of A*/A grades achieved by pupils at A level. GCSE statistics can be misleading in state schools since some of the exams reported as GCSEs could be BTECs and also because a state school's GCSE's results can depend on how many pupils are from deprived backgrounds /on free school meals, how many don't speak english as a first language etc. Independent school results are freely available on their websites and league table are compiled by the likes of the Sunday Times and various others -see eg www.best-schools.co.uk/uk-school-league-tables/
The national average of state schools is around 26% A/A at A level but in some areas it is much higher and there are non-selective state schools which have around 50% A/A grades at A level . Indeed a surprisingly high number of the local private schools in my area had worse A level results last year than the better state schools. To me this would be a red flag i.e. if a local private school has worse A levels than a local non-selective state school then you would have to think carefully before spending your money...

Visit the schools. Do you like the atmosphere, the ethos, the behaviour?
Do they do the subjects your Dc wants to do? The particular languages, etc. Do they have what you are looking for in terms of music provision?

Do they have children of the same academic potential as yours? Is there some indication that children of all abilities are being challenged, and supported, do the exam outcomes look ok.

Ranking and meticulous statistical comparison is pointless. 2 students who get overtired and anxious and get (shock horror) B’s instead of As will reduce the % stats. Some schools fiddle the stats: see the current thread where a school is entering a child as an external candidate because she has missed a term for illness. The statistical comparison will tell you that it is THE top girls school!

Your child will do best where they feel happy and confident. A table of comparitive stats won’t tell you that.

rank them not ran them!

if you want to ran them according to exam results most independent schools publish their exam results on their websites, with this info you can then devise you rank them yourself. I'm no maths mathematician but even I could do this if I wanted to. Obviously you have to take into consideration how selective they are, but then you'd have to do that in the state sector. Do look at the actual subjects as well especially for A levels . 89% getting A*'s is more impressive if every subject is a very academic one you could also look at university destination if they are published, although to get an accurate picture you need to know how many are in the 6th form if for example a schools 15 went onto Oxbridge last this is impressive if there are only 90 pupils in yr 13 less impressive if there are 250.
If its value added or % from outside of the UK you want you could look at ISI reports although IMO they are fairly worthless and this info is not always readily available.