The National Alliance of Public Charter Schools estimates in a new report that 2.9 million children now attend U.S. charter schools, up 9 percent from the last school year.

Take that with a grain of salt: The same organization estimated a year ago that enrollment had already reached 2.9 million, a figure that turned out to be off by a couple-hundred-thousand students. So in 2015, charter school enrollment didn’t grow by 14 percent, as the National Alliance (and The Washington Post) reported, but by closer to 7 percent.

A spokeswoman attributed the mis-estimation in part to a quirk in Arizona policy that encouraged traditional schools to convert to charter schools, and that didn’t result in as many conversions as it initially appeared. The organization also changed its estimation method, going school by school to better pick up on unreasonable growth rates.

The 7 and 9 percent increases over the past two years were lower than the average 11 percent annual enrollment growth over the past eight years, according to the National Alliance.

The organization’s estimates — though imperfect, as estimates tend to be — provide the closest thing to real-time enrollment data for charter schools nationwide. (The latest federal data on charter school enrollment, for example, appears to be from 2012-2013.) This year’s report says that more than 400 charter schools opened their doors for the 2015-2016 school year, and another 270 stopped operating. There are now more than 6,800 charter schools in 42 states and the District.

“The continued expansion of charter public schools nationwide, year after year, demonstrates the demand for quality public school options,” Nina Rees, president and CEO of the National Alliance, said in a statement.

This map shows how enrollment varies by state, according to this year’s National Alliance estimates:

This map shows how total enrollment also varies widely by state: