Ofsted has not done enough throughout its history to test the reliability of its inspections, according to the body’s national director for schools.

Writing in response to a critical blog posted by headteacher Tom Sherrington last month, Sean Harford said that one of the reasons behind this is down to the focus on quality assurance.

“This provides assurance that the process is carried out consistently as we would wish, but not directly that different inspectors in the school on the same day would give the same judgement,” Mr Harford elaborated.

“I have built in reliability testing for the pilots for the new short inspections this term. If reliability is a problem, we will review the issues to see what we need to do to make the inspections reliable.”

These comments were focused on Mr Sherrington’s assertion that Ofsted’s inspection system borders on the “farcical”, so much so that no school can be judged in a “meaningful way”.

The head of Highbury Grove School in Islington added that there is something inherently wrong with the process, which takes, he argues, swathes of quantitative and qualitative data and “distils” it into a “simple set of final grades”.

“The extent to which we accept this in our system despite the enormous flaws and the absence of proper validity trials continues to astonish me,” he said in the blog on his own personal website.

Speaking to the BBC, Russell Hobby, general secretary of the National Association of Head Teachers, stated he welcomed Mr Harford’s comments but remained unsure about Ofsted’s ability to be reliable.