That's partly because charters remain a predominantly urban phenomenon, the researchers found, with charter schools three times as likely as regular public schools to be in located in a big city.
"One of driving forces behind charter schools has been parent demand for new options among groups that seemed to be less-well served by traditional schools," said author Robin Lake, director of the National Charter School Research Project at the University of Washington's Evans School of Public Affairs.
Overall, the charter experiment launched in the early 1990s continues to expand, researchers found, and now educates more than a million children in 40 states and the District of Columbia. The number of schools added in the 2004-5 school year was one of the largest in the movement's history, Lake said.
But the researchers also found that the recent growth is concentrated in only seven states, with much of the nation maintaining strict limits on charter schools.
Overall, the report concludes that the 2004-5 school year was a "tumultuous" one for the movement, marked by headlines about conflicting studies on whether charter schools boost student performance or hinder it.
"There is not a lot of reliable research, and a real mismatch between what the available research can tell us and what policymakers and citizens need to know," said co-author Paul Hill, a research professor at the university's Evans School of Public Affairs. "That is why this project was created - to provide comprehensive and objective research."
Community, parent or teacher groups seeking new public-education options generally obtain charters with the approval and oversight of a local school district or other state agency, and receive public funds based on the number of children who attend.
With charter policies diverging markedly from state to state - and most of the research to date assembled by advocacy groups rather than independent scholars - the UW researchers asked state officials to provide data that had never before been compiled. The resulting 83-page report, titled "Hopes, Fears, and Reality: A Balanced Look at American Charter Schools in 2005," reveals that charter schools:
- Serve a higher percentage of minority students than non-charter schools - 58.5 percent versus 45 percent - a key point for boosters who promote charters' potential for helping groups ill-served by traditional schools. Charters' minority enrollment was strongly linked to their urban locations.
- Enroll a bigger share of low-income children than regular public schools. More than half (51.7 percent) of charter students are eligible for subsidized lunches, compared with 39.9 percent of non-charter kids.
- But serve a smaller proportion of special education students than do regular public schools - 10.8 percent of charter students are so classified, compared to 13.4 percent of non-charter students.
- Have, on average, about half as many students as the typical public school.
- And are three times more likely than regular public schools to diverge from the traditional age/grade divisions of elementary, middle and high school. Many charters, for example, cover grades K-8.
"They're basically doing away with the notion of middle school," said Lake, "which is an appealing concept to some parents."
But national trends found in the study were punctuated by stark variations from state to state in nearly every category.
"This study makes clear that it is nearly impossible to generalize about charter schools as a national phenomenon," Hill said. "Charter schools are really a state and local policy tool that can be used well or badly."
In addition to compiling and analyzing data, the report also explores pressing issues such as measuring student achievement, replicating successful charter schools, handling charter school closures and making "apples-to-apples" comparisons in funding.
The report will be presented at a Nov. 21 policy luncheon at the Urban Institute in Washington, D.C.
The National Charter School Research Project was established at the University of Washington's Evans School in fall 2004 with support from a consortium of foundations convened by the Philanthropy Roundtable.