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Forty states, along with the District and Puerto Rico, have authorized about 4,600 charter schools, serving an estimated 1.4 million children. Under the Obama stimulus plan, about $650 million is available to states under the education department’s discretionary Innovation Fund, with an additional $211 million directed at charter-specific programs, according to the National Alliance for Public Charter Schools.

The Stanford study was criticized by a charter proponent, the Center for Education Reform in Washington, which disputed the research methodology and said the analysis did not fully address differences in state charter laws.

“The CREDO report fails the most important and most objective test of student data analysis through their use of virtual twins to replicate real student growth by creating ‘straw men’ subjects,” CER President Jeanne Allen said. “This suggests that virtual methodology can overcome comparative analysis by making the study world look real, act real and sound real.”

The center questioned the CREDO findings that said students in poverty showed gains, while black and Hispanic students were found to be lower-performing, despite the higher rates of poverty in those groups.

“Well-documented and nationally recognized survey work representing more than a third of all charters finds such demographics to be one and the same,” the CER said.