reported concerns that the inspection regime was leading to increased workload for teachers because of a new focus on marking (" `Assessment treadmill' may overwhelm staff", 14 November).
And earlier this year, a damning report by the Policy Exchange thinktank claimed you would be "better off flipping a coin" than trusting an inspector's verdict on a lesson.
Jonathan Simons, head of education at the thinktank, said the latest findings called the "legitimacy" of the inspectorate into question. "If Ofsted can provide the data that shows all of its inspectors are well-trained and all of their judgements are valid, I would accept the variation," he said. "But at the moment I don't have any confidence that all individual inspectors are making the right judgements.
"That's a real problem for the legitimacy of Ofsted. It's yet more evidence to support the theory that the grade you get depends on who arrives at your school."
Stephen Ball, principal of New Charter Academy in Ashton-under-Lyne, Greater Manchester, said the figures suggested that inspection outcomes came down to the "luck of the draw". He added: "It seems to point to real weaknesses in quality assurance in terms of inconsistency between inspection teams."
Russell Hobby, general secretary of the NAHT headteachers' union, said the differences could stem from some inspectors being sent into a high proportion of challenging schools or working in low-performing areas. But the variation in grades was still a cause of pressure on teachers, he added.
"Inconsistency in Ofsted inspections leads to overcompensation in schools," Mr Hobby explained. "If you don't know which inspector you're going to get, you tend to find people overdoing things to cover every possible base.
"That's where the fear of Ofsted arises: it's not the fear of the basic framework, which very few people are saying is unreasonable. It's fear of how it's going to be applied."
Brian Lightman, general secretary of the Association of School and College Leaders, called on Ofsted to ensure that its framework and quality assurance processes were applied consistently. "Both we and Ofsted know that there are still too many inconsistencies. These variations need to be addressed," he said.
An Ofsted spokesman said: "We undertake around 6,000 school inspections each year. More than nine out of 10 schools are satisfied with their inspection outcome, while a survey of 850 schools inspected earlier this year found that more than eight out of 10 believed that the inspection process had helped them to improve.
"We are always looking to enhance our inspection process. In less than a year all additional inspectors will be contracted directly by Ofsted so that we will have more direct control over their selection, training and quality assurance."
`The regime is demonstrably inconsistent'
New Charter Academy in Ashton-under-Lyne, Greater Manchester, was judged to be requiring improvement last year. Principal Stephen Ball believes another visit from inspectors is imminent.
The findings of the research are "shocking", he says. "On the face of it, it appears to confirm the belief that so many of us have: that inspection is not producing consistent outcomes."
"I'm not impugning the integrity of the individual inspectors concerned: they're managing a framework which is in itself flawed, which changes too frequently for people to master.
"Probably, with Ofsted's quality assurance of inspectors not entirely doing its job, that's why we're getting these highly questionable results.
"Nobody believes it is acceptable for a national inspection regime to be so demonstrably inconsistent and for it to depend on individuals. For those of us in challenging circumstances, this is even more worrying."