Criticism of Ofsted has been “overdone” and used by schools “as a way of deflecting attention away from underperformance and failure”, the head of the watchdog claims.

Writing exclusively for TES, chief inspector Sir Michael Wilshaw argues that accusations of inconsistency among inspectors have become “a Pavlovian response to unfavourable inspection outcomes”.

Schools disappointed with their grades should take a “long, hard look in the mirror” and not “shoot the messenger”, he says.

Sir Michael hit back after TES revealed last week that 49 of Ofsted’s most prolific lead inspectors had not judged a single school outstanding this year (“Does your grade depend on the inspector you get?”, 21 November).

Data produced by the Watchsted website, which collects up-to-date information on inspection results and inspectors’ verdicts, also shows that more than 100 inspectors who have led at least 10 routine inspections in 2014 had not yet rated any schools inadequate. Headteachers claimed that the figures suggested a school’s inspection grade was partly down to the “luck of the draw” on which lead inspector they were assigned.

In today’s TES, Sir Michael admits that examples of “poor practice” have been found, adding that “a number” of externally contracted lead inspectors have been demoted for this reason.

However, he writes: “I believe criticism of Ofsted has been overdone and has been used as a way of deflecting attention away from underperformance and failure. Too often, the charge of inconsistency has become a Pavlovian response to unfavourable inspection outcomes.”

He adds: “I appreciate how hard it is to see colleagues who give so much to the school judged coolly by strangers who inspect for a couple of days and find them less than perfect. Teachers work with passion. Inspectors have to deal in detachment. It can make for an uncomfortable cultural fit.

“But that doesn’t mean the default position should be to condemn the judgement given - or the person delivering it. Too often people find it easier to shoot the messenger than to take a long, hard look in the mirror.”

Sir Michael calls on leaders of good and outstanding schools to sign up as inspectors on a full- or part-time basis, in order to help the profession “take more ownership” of inspection.

He acknowledges that Ofsted “doesn’t always get it right” but insists that the body’s quality assurance procedures are “robust” and identify poor practice. “A number of additional inspectors have had their lead inspector status removed after concerns about performance,” he writes.

The chief inspector also insists that inspection teams are not “deployed randomly on a `taxi rank’ basis”, adding: “Individuals with the right skills and expertise are chosen to inspect relevant schools.”

Brian Lightman, general secretary of the Association of School and College Leaders, said that Ofsted’s move to end contracts with independent service providers and bring all inspectors in-house from next September amounted to recognition that the current system “is not working”. He added: “As far as we’re concerned, the sooner we have a system of inspections led by [in-house inspectors], the better.”

Stephen Ball, principal of New Charter Academy in Greater Manchester, has experienced more than a dozen inspections as headteacher of several schools. “I would urge Ofsted to look at the data about inspectors and produce a proper analysis - in the same way a school would be required to analyse its own performance - and then take action to iron out variations between different departments and teams,” he said.

Read the full comment piece in the 28 November edition ofTESon your tablet or phone or bydownloading the TES Reader appfor Android or iOS. Or pick it up at all good newsagents.