Iain Duncan Smith has argued for a voucher-style school 'credits'

Researchers looking at voucher schemes in Cleveland and Milwaukee "found little or no statistically significant differences in voucher students' achievement test scores compared to public school students".

But this was qualified by researchers' finding that there was often insufficient data to make complete comparisons - and that more research will be needed to assess any impact on student achievements.

The research was carried out on behalf of the US General Accounting Office, which scrutinises public spending on behalf of Congress.

Under the voucher scheme, parents are entitled to state funding which they can use to "buy" school places for their children.

This can mean parents using state funds to pay for a place in private schools as well as to change schools within the state sector.

This would also include religious private schools - and a series of cases are due to be heard by the Supreme Court next year to consider whether the state funding of religious schools is in breach of the US constitution.

Free market

The calls for vouchers has been a policy associated with free-marketeers, who have argued that parents need to have greater consumer choice.

Opponents have argued that such schemes will subsidise private schools at the expense of weakening the state sector and that it will leave inner-city state schools, which many pupils will still have to use, in an even more neglected condition.

But there have also been claims that such vouchers would be of particular value to low-income and ethnic minority families, whose children are trapped in underachieving schools

Vouchers, it has been argued, would allow parents to take their children out of an environment of deprivation and send them to private schools used by more affluent families.

Both the Cleveland and Milwaukee projects have been focused on low-income families.

In Cleveland, families can receive annual vouchers worth up to 90% of school tuition, up to a maximum of $2,250 per child. And in Milwaukee, parents could receive the full cost of fees at private schools, up to a maximum of $5,106 per child.

In the year, 1999-2000, this meant that state funds worth $39m a year were passed to private schools participating in the voucher scheme.

Parents

But the findings of the research have so far found little evidence that this use of taxpayers' money is producing better results.

Parents, when given a vote on the question, have so far been lukewarm about school vouchers.

Last autumn, after much advertising and public debate, a proposal to provide vouchers for all parents in California was heavily defeated in a referendum, suggesting that even Republican-supporting parents were unconvinced by the policy.