IMAGINE you are a poor parent in Washington, DC. You assumed you would send your child to a public school. But you have been offered a voucher worth up to $12,000 towards tuition at a private one. Should you use it?

Until recently the evidence suggested that you should. In 2004 Congress created the DC Opportunity Scholarship Programme, the first school-voucher scheme directly subsidised by the federal government (states and charities subsidise many others). Since then up to 2,000 families a year have been handed vouchers to attend private school after winning lotteries. In 2010 a study found that 82% of pupils offered a voucher went on to graduate from high school, compared with 70% of similar peers who attended public schools.

Studies of Milwaukee, which introduced vouchers in 1990, have found similar effects on graduation rates. A 2015 study of a privately funded programme in New York found that blacks who received vouchers had higher rates of college enrolment. In Vermont the value of a house is higher if it is in an area that offers school vouchers, suggesting that parents will pay to become eligible for them.

Yet evidence is piling up on the other side. In the past two years studies of Louisiana and Ohio have found that pupils using a voucher did worse on state tests than peers at public schools. A recent literature review concluded that “the effects of vouchers have been disappointing relative to early views on their promise”.

On April 27th another study put the boot in. The Institute of Education Sciences, the research arm of the Department of Education, analysed the results of children in Washington, DC’s scheme between 2012 and 2014. It found that, on average, pupils who attended private school had lower maths scores at the end of their first year than those who did not.

Children often take time to adjust to new schools. Still, the results seem disappointing for advocates of school vouchers, a group that includes many Republican governors and Betsy DeVos, Donald Trump’s education secretary.

They nevertheless suggest that other education reforms are working. The relatively lacklustre performance of private schools in Washington, DC reflects improvement in public schools. A decade ago Adrian Fenty, then the mayor, took away powers from the city’s elected board of education and installed a new schools chancellor, Michelle Rhee.

She began to hold teachers at traditional public schools accountable for pupils’ performance. The worst were fired and the best earned bonuses and pay rises. The city also encouraged the growth of independently-run public schools, known as charter schools. In 2015-16, 45% of pupils in public schools in the city were at charters, up from 24% a decade previously.

Meanwhile private schools, many of which are attached to churches, have struggled. In America as a whole, the Catholic ones have swapped nuns for professional teachers and have been hit financially as dioceses have paid compensation for historical sexual-abuse cases. Competition from charter schools has suppressed enrolment, too. Good statistics are lacking, but the Urban Institute, a think-tank based in Washington, estimates that the number of pupils aged between five and 17 in private schools may have dropped by two-thirds between 1999 and 2014.

To survive, several Catholic schools in Washington have shed their religious affiliation and converted to charter schools, thereby adding pupils to the public rolls. Overall enrolment in public schools has increased since 2008-09, from 70,919 pupils to 87,344 in 2015-16. Most of that growth was a result of swelling attendance at charter schools, but traditional public schools expanded, too.

Between 2011 and 2015 traditional public schools in Washington made larger gains in the National Assessment of Educational Progress, a nationwide set of tests, than those in any other large city. Pupils at charters tend to score even higher on citywide tests than those at other public schools. Gentrification is one reason for the increased scores, notes Matthew Chingos of the Urban Institute. But his research also shows that the gains are larger than demographics alone would suggest.

The latest study indicates that, at least initially, vouchers are only as effective as the schools they allow children to attend. But it does not undermine the argument for competition, since the pressure from charters is one reason why public schools have improved. A parent with a voucher may increasingly think twice about using it. That is a good choice to have.