Ofsted has not done enough to ensure school inspections are reliable, a senior official has admitted.
Sean Harford, the watchdog's national director for schools, was responding to a critical blog from a head teacher.
Some inspectors use data as a "safety net" instead of making a professional judgement, Mr Harford wrote.
Head teacher Tom Sherrington had complained of "enormous flaws and absence of proper validity trials" in the current inspection system.
In the blog, Mr Sherrington, head of Highbury Grove School in north London, had rejected the idea that schools "can be judged in a meaningful way via inspections".
"By 'judged' I am not talking about an experienced visitor giving some insightful developmental feedback based on an analysis of the available data and their observations; no doubt there are some people out there who can do this well enough.
"I am talking about the process of distilling this mass of qualitative and quantitative information into a simple set of final grades, with one overall Judgement Grade," wrote Mr Sherrington.
In his response, Mr Harford admitted Ofsted does not currently ensure "directly that different inspectors in the school on the same day would give the same judgement".
He also agreed "some inspectors and some schools focus too much on a narrow range of data".
He said Ofsted trained its inspectors to use data as a "signpost", rather than making it a "pre-determined destination".
"But the weakest ones have been guilty of using the published data as a safety net for not making fully-rounded, professional judgements."
He said inspectors should draw on information from the school itself, including pupils' work over time, progress across year groups and classes, improvements to teaching, and pupils' attitudes to learning.
Mr Harford said Ofsted's pilots for its new short inspections would include reliability testing.
"If reliability is a problem, we will review the issues to see what we need to do to make the inspections reliable."
Last month the watchdog closed its consultation on a new inspection framework which will introduce shorter inspections for all schools rated as good in England.
School leaders have described Mr Harford's comments as a definite shift in tone.
Mr Sherrington told the Times Educational Supplement he was amazed by the response to his blog, arguing that inspectors "can't just continue to assert authority - they have to demonstrate reliability".
Russell Hobby, general secretary of the National Association of Head Teachers, welcomed Mr Harford's comments but said he was a sceptical of Ofsted's ability to ensure reliability.
"They don't set out what steps they are going to take to make inspections more reliable."
Mr Hobby pointed out that less reliance on data would mean "you would need really highly-trained teams that really know what they are doing".
He said under the current system, some inspectors had made up their minds before arriving at the school, based on league-table data.
"That is a big waste of money and an insult to teachers."