Schools face being scrutinised twice on the same day by different Ofsted inspectors under reliability testing being piloted by the watchdog this term.

As revealed in TES last week, Ofsted has acknowledged for the first time that it has failed to ensure inspection judgements are reliable. A senior official at the watchdog said it could not guarantee that “different inspectors in the [same] school on the same day would give the same judgement”.

Sean Harford, Ofsted’s national director for schools, said that the organisation was carrying out reliability testing in pilots of shorter inspections taking place this term, in order to asses consistency between different teams of inspectors.

The inspectorate has now revealed the process will involve two Her Majesty’s Inspectors (HMIs) - senior officials employed directly by Ofsted - “independently inspecting the same school on the same day and comparing the judgements”.

“To test the reliability of the new short inspections, we conducted consultative trials in schools in the autumn term,” a spokeswoman said. “Feedback was positive and we will carry out pilots to test the shorter inspections across a range of schools in the spring term.

“Reliability of the short inspection methodology will be tested during the pilots by two [HMIs] independently inspecting the same school on the same day and comparing the judgements.”

Mary Bousted, general secretary of the ATL teaching union, said she had no confidence in Ofsted’s ability to assess the reliability of its own judgements, and called for its work to be externally verified.

“For a school, being inspected twice in one day would be a daunting prospect. But at least, for the first time, they might receive a judgement that has been properly moderated,” she said. “Ofsted’s reliability has been challenged since its inception in 1992. For it to have taken this long to address [the concern] shows what an unreliable indicator of school performance it is.”

Ofsted’s frank acknowledgement of its shortcomings came in a written response by Mr Harford to a blog by headteacher Tom Sherrington. In the statement, Mr Harford acknowledged that the “weakest” inspectors “have been guilty of using the published data as a safety net for not making fully rounded, professional judgements”.

“If reliability is a problem, we will review the issues,” he added.

Mr Harford also hit back at Ms Bousted’s comments, and said that Ofsted already goes “to great lengths, through our existing quality assurance process, to ensure that inspectors make judgements which are rigorously based on the evidence gathered”.

“It is frustrating that our attempts to enter a genuine debate about how we might improve still further our approach are being used to score cheap points about Ofsted’s work,” he added.

Related stories:

Ofsted inspections are unreliable and too often based on data, watchdog admits - January 2015

‘Ofsted’s criteria for “outstanding teaching” are outstanding nonsense, and here’s why’ - January 2015

Tristram Hunt: Ofsted must move beyond ‘box ticking and data dependence’ - January 2015