Primary schools are no longer to be assessed on standards in music or any other non-core subject.

The latest guidance to school inspectors from the Office for Standards in Education says they should concentrate on literacy and numeracy in primaries and stop grading attainment in art, design and technology, geography, history and PE, as well as music.

Chris Woodhead, the chief inspector, wants his staff to make greater use of national curriculum test results. In cases where inspectors’ own judgments on standards differ from the levels suggested by performance data, they should explain their conclusions.

At primary level, there are national test results only in maths, English and science. From September, inspectors will only be required to grade those subjects, plus religious education and information technology.

The guidance says schools are expected to adopt the National Literacy Strategy, and inspectors’ reports will be expected to evaluate the impact of the schools’ strategy on the quality of teaching and standards achieved.

The emphasis in the new guidance on inspectors having regard to national test results comes at the same time as OFSTED publishes its own research on discrepancies between inspection reports and test results.

The paper says there is a “broad match” between assessments made by inspectors of attainment at the top end of primary and national test results. A perfect match is not expected, says the paper, because the test may have been taken some time before the school inspection. However, OFSTED accepts that inspectors find it hard to classify performance in small schools; half of primary schools have under 30 11-year-olds taking national tests.

OFSTED’s interpretation of its data is rejected by Don Foster, the Liberal Democrats’ education spokesman. He says the figures show inspection gradings and test results are not broadly in line. In primaries scoring well above the national average, only 60 per cent were highly rated by OFSTED. Where schools scored poor test results, less than 50 per cent were rated well below average by OFSTED.

As a result of its research, OFSTED will advise inspectors on how to interpret test data and the need to explain any reported differences between inspection evidence and these data.

Inspectors will from September be required to make full use of “increasingly reliable indicators”. Training is to be provided in judging attainment and progress and interpreting data.

However, Mr Foster believes the insistence that inspectors make use of test data casts doubt on the inspection system.

“If we are saying inspection judgments are only accurate when there is reliable test data to base them on, why do we spend millions sending OFSTED inspectors round schools?” he asks.