An American study on the use of high-stakes tests has concluded that they have not had a positive effect on student achievement.

The report from the National Research Council, a respected independent body, says that programmes which impose sanctions or offer rewards on the basis of test results can encourage teachers to teach to the test.

It goes on to call for schools to be measured by a range of student outcomes - for example, looking at average test scores rather than a pass line would overcome the problem of teachers focusing on borderline children.

The study also points out that when incentives encourage teachers to focus on the material included in a test, understanding of the untested portion of the subject may decrease.

For this reason it is important to broaden the range of material included in tests where feasible and look at more sophisticated ways of using incentives.

The report originated in the debate on the No Child Left Behind Act, which required students to pass a state-wide standardised test and set in place a series of interventions for those who did not reach their annual targets - which, ultimately, could mean the closure of the school.

President Obama is currently trying to have the act reformed with a review system that would give states and school districts more responsibility over schools.

The report comes as the Bew report into key stage 2 assessment in England recommends that rolling three-year averages are included in the performance tables and more use is made of teacher assessment.