Students with disabilities are showing improvements in math and reading in more states than not, though these students continue to lag far behind their peers without disabilities, a new report finds.

From 2006 to 2008, average reading and math test scores rose for elementary, middle and high school students with disabilities, according to the report from the Center on Education Policy. Meanwhile, the percentage of students with disabilities who scored at basic, proficient and advanced levels rose in most states. Nonetheless, students with disabilities remain as much as 30 or 40 percent behind their typically developing peers.

The report is based on state assessments used to determine adequate yearly progress under No Child Left Behind. But researchers warn that it’s hard to gain a clear understanding of the progress made by students with disabilities. That’s because there are varying numbers of such students in each state from year to year and it can be difficult to assess the validity of alternative tests offered to students with disabilities in some states.

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“The differences among states in approaches for testing students with disabilities and reporting results make it hard to obtain a clear picture of achievement for students with disabilities,” said Jack Jennings, president and CEO of the Center on Education Policy. “Federal and state policymakers should take steps to clarify how results for students with disabilities on state tests are reported so that trends will be more valid and meaningful in the future.”