WASHINGTON — Math and reading test scores are up in most states since the No Child Left Behind law took effect in 2002, but it's impossible to know how much credit the law deserves, a new report says.

In an exhaustive study released yesterday, the Center on Education Policy also concluded that the historically wide achievement gap between black and white children has generally narrowed in many states — exactly what NCLB supporters said they wanted to achieve when President Bush signed the law.

But the law's contributions are hard to measure because a number of states already were taking steps to boost reading and math, the study's authors say. And because every public school falls under the law, there is no group of students to use for comparison, they said.

What the law clearly has done — the change some identify as its most notable benefit — is give researchers and parents the data to track student progress. By requiring testing in math and reading every year from third through eighth grade and once in high school, the law provides a wealth of information about a school's performance over time.

"We know a lot more about student achievement because of No Child Left Behind," said Jack Jennings, president of the Center on Education Policy. "There's just much more testing and much more information than ever before, so you can reach sounder conclusions."

The report took score results from reading and math tests over several years and determined progress not only by examining the portion of students who scored at the "proficient" level — the target established under No Child Left Behind — but also the portion of students who showed any progress, even if they didn't pass the test.

Researchers also measured a state's progress against the National Assessment of Educational Progress, the federal math and reading test taken by a small percentage of students in each state.

Test scores may be rising, but that's at least partly due to states lowering standards to meet the law's demand that all students become proficient in both subjects by 2014, said Bruce Fuller, a professor of education and public policy at the University of California at Berkeley.

"They lower the bar and design tests that are highly sensitive to slight gains for low-achieving students," he said. "Progress is being made but not anywhere near the rate claimed in this report."

Learn more:

Center on Education Policy: www.cep-dc.org