Schools face double inspections to ensure reliability
Some schools in England could be visited twice on the same day by different Ofsted inspectors to test the reliability of findings.
The watchdog plans to pilot double inspections this term, with inspectors comparing judgements to see if they have reached the same conclusions.
If effective, the method could be included in a new inspection framework for "good schools" from September.
Teaching unions said questions about reliability had always "dogged" Ofsted.
A week ago, Sean Harford, Ofsted's national director for schools, admitted not enough had been done to ensure reliability.
Responding to a critical blog from head teacher Tom Sherrington last week, Mr Harford admitted Ofsted does not currently ensure "directly that different inspectors in the school on the same day would give the same judgement".
In the response, Mr Harford said some inspectors used data as a "safety net" instead of making a professional judgement.
Ofsted's consultation on new short inspections for good schools closed late last year.
This term, pilot inspections for the new framework will include investigating new methods of reliability testing, said Mr Harford.
Ofsted now says this will involve two senior officials independently inspecting the school on the same day and comparing judgements.
"There will be a small number of pilots this term to investigate whether two inspectors come up with the same conclusions," a spokeswoman told BBC News.
The spokeswoman said there was no suggestion the method would be used for all the shorter inspections from September but could instead "be an occasional feature as part of the quality assurance process".
Teachers' unions said they had been questioning the reliability of Ofsted's judgements for years.
Kevin Courtney, deputy general secretary of the National Union of Teachers, called for "root and branch reform" of school accountability in England, including the abolition of Ofsted.
"The suggestion that inspecting schools twice will resolve the flaws in the current system is nonsense.
"In effect, this is merely punishing schools for Ofsted's failings."
Mary Bousted, general secretary of the Association of Teachers and Lecturers, said questions about reliability had "dogged Ofsted since it was set up in 1992".
"It is incomprehensible that Ofsted has waited 23 years to investigate whether or not its judgements are reliable when the consequences for schools are so devastating if they are judged to be poor.
"We note that Ofsted, however, is not opening itself up to external evaluation of the reliability trials.
"If Ofsted has robust quality assurance, why is it not opening itself up to external scrutiny?"
Sean Harford said the quality of its inspection judgements was "of the utmost importance to Ofsted".
"We go 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 in our approach are being used to score cheap points about Ofsted's work."
The Ofsted spokeswoman said quality assurance measures already in place included site visits by senior inspectors to test evidence, data analysis and the reading of all inspection reports to ensure the text matched the overall grade.