American students are regressing in math, and Common Core is to blame, says a scholar from the centrist Brookings Institution.

States whose students took Common Core tests in the 2014 to 2015 school year scored worse on the Nation’s Report Card in data released Wednesday, according to Brookings Senior Fellow Russ Whitehurst. “The correlation between Common Core assessment participation and changes in [National Assessment of Educational Progress] is -.35,” Whitehurst writes. The differences are statistically significant.