The results of inspections, which are used by many parents to choose a school, have been condemned as “superficial”, “worthless” and a waste of money.

A study published today claims that inspections by the Office for Standards in Education (Ofsted) have become so short they uncover little useful information and rely too much on exam results and on head teachers’ assessments of performance.

The study by Civitas, a think tank, comes ahead of this week’s annual report by Ofsted, headed by Christine Gilbert, wife of Tony McNulty, the employment minister.

It is likely to alarm many parents who use Ofsted reports to help them make decisions about their children’s future. The results of good inspections are also sometimes publicised by estate agents to market property and can help to lift prices in areas surrounding high-performing schools.

“Most parents have total faith in Ofsted, a faith that is worryingly misplaced,” said Anastasia de Waal, editor of the study. “Its judgments are not reliable guides to quality.”

Civitas claims that the lack of detailed information from the shortened inspections has led to excessive reliance on test and exam results.

Research by The Times Education Supplement, which is included in the report, indicates that Ofsted school gradings are the same as those based on exam results in almost all cases. “Spending tens of millions on essentially regurgitating exam results is clearly taxpayers’ money misspent,” says Civitas.

Declining confidence in the inspections may boost the popularity of independent league tables such as today’s Sunday Times Parent Power rankings. Its top performers for 2008 include St Olave’s grammar in Orpington, Kent, state secondary of the year.

Ofsted, with a £236m budget, is increasingly powerful. In recent years its remit has been expanded from inspecting state schools to areas such as adult education, training and nurseries. It is also gaining increasing powers over independent schools.

In 2005, however, Ofsted inspections were shortened, partly to make them less of a burden on teachers. Before then, up to 18 inspectors would visit a school for four or five days; this has been cut to two days by four or five officials.

Sarah Drake, a serving inspector who contributed to the study, said: “Reduced time means it is not always possible to unearth real problems.” She warns of a tendency towards inspectors “agreeing with the school lest disagreement should lead to a complaint”.

The schools department said: “Civitas pretends to take an objective view but has based its conclusions on conjecture and the opinions of interested parties.”

Ofsted said: “They do not understand how inspection works . . . inspectors make more than 30 judgments. Only one relates directly to exam or test results.”

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