Schools' inspectors in England have been sent new guidance after complaints about a sharp rise in the number of schools failing inspections.
"Balance of judgement" is needed, says David Bell
Ofsted's chief inspector, David Bell, has questioned the judgements made by some inspectors.
He suggests they may have misinterpreted a new inspection framework brought in last September.
Some head teachers have complained that the changes meant good schools were being classed as failing.
The rule change meant schools could be said to be failing if 10% of lessons were unsatisfactory, whereas previously the figure was 20%.
In the autumn term of 2003, the number of schools said to need "special measures" was 90, up from 54 in the previous autumn.
Now David Bell has written to inspectors to tell them they need to make sure they are using balanced evidence and should not rely too much on the grades given to lessons observed by inspectors.
"Too many letters suggest to me that inspectors and inspection teams are not always getting the balance of judgements quite right," he wrote.
The guidance also said inspectors should "celebrate a school's main strengths as well as diagnose its weaknesses".
Initially, the rise in the number of failing schools was attributed to the tougher inspection regime, for which David Bell had said there would be "no apology".
He said expectations of performance should rise.
In September, another new inspection framework is being introduced, which will involve schools being inspected at short-notice, more often.