GOV. GEORGE PATAKI, Mayor Michael Bloomberg, Chancellor Joel Klein and other New York officials are in a huge hurry to raise the state limit on the number of charter schools, now set at 100. Bolstered by a growing demand for these more autonomous schools and some positive charter school test results in the city, they seem to think that the more charter schools, the better for the children of our city and state.

But recent research suggests that not all charter schools are created equal, and that New York policymakers should focus more on the quality of existing charter schools and on how to support the good ones before they call for an expansion.

After Minnesota passed the first charter school law in 1991, the idea of creating publicly financed schools that were free from regulation but held accountable for student performance became popular. Within 15 years, 40 states and the District of Columbia had passed charter school laws. Today there are an estimated 3,600 charter schools enrolling one million students nationwide.

Over the years, the charter school concept has appealed to a wide spectrum of people, including free-market reformers, advocates of testing to measure schools' performance, and progressive educators and parents. Indeed, in 1998 when New York passed its charter school law, in the headiest days of the movement, these schools were supposed to offer something for everyone. Their supporters promised they would increase student achievement, choice and innovation.

But today, while the public's attitude toward charter schools remains mostly positive and many strong charter schools are up and running, those of us who study research findings realize that the promises charter school supporters made nearly a decade ago haven't been fulfilled.

Whether we look at national or state test scores that measure student achievement, the results for charter schools in comparison to similar public schools nearby are mixed at best. In many cases, when comparing schools that have similar student bodies, researchers find the regular public school students doing better than those in charter schools.

After the American Federation of Teachers released a report in 2004 showing that charter school students tended to have lower scores on a national test than similar students in public schools, charter school proponents chalked up these findings to the anti-charter bias of the teachers union. But then the United States Department of Education examined the same data and came up with similar results several months later.

Indeed, if last spring's New York City test scores for charter schools were subjected to a more sophisticated analysis that took into account the backgrounds of students in charter and public schools, it isn't clear that charters would fare as positively as they did on the state's eighth-grade exam. Meanwhile, the scores for fourth graders in charter and public schools in the city were similar.

In addition to this wobbly achievement data, there is little evidence that charter schools are being held more accountable for student achievement than are regular public schools. In this era of No Child Left Behind, when all schools are subject to strict sanctions for not making adequate yearly progress toward helping all students achieve proficiency on state exams, charter schools have no special claim to accountability.

And as it turns out, charter schools are more likely to have their charters revoked for fiscal malfeasance than they are for low test scores. Last year, fiscal and legal concerns impelled California to close a chain of 60 charter schools serving 6,000 students.

If that is not enough bad news to dampen New York lawmakers' enthusiasm for creating more charter schools, there is mounting evidence that these schools serve as new kinds of sorting machines, leading to more racial or class segregation within local communities. We also know that charter schools overall are far less likely to serve the growing numbers of special education students or students who don't know English, leaving local public schools to serve larger percentages of these students.

And when it comes to innovation, research suggests that charter schools are more innovative in using public funds to support private ventures than they are in instructional practices. This may please free-market supporters of charter schools who have defined ''strong'' charter laws as those that foster large numbers of charter schools regardless of the quality of those schools.

The focus on quantity encourages private investors to try to make money by running substandard charter schools and skimming off public money for themselves. Such was the case in California when the chain of schools was shut down. This is not the first time this has happened in a ''strong'' law state, and when it does children and state taxpayers are the victims. I have studied charter schools in California and urge my fellow New Yorkers not to support a law that could open the door to the proliferation of such institutions.

Indeed, New York should lead the way in redefining charter school reform as being not about quantity, but about quality. The state, especially New York City, has a rich history of public school choice -- alternative and magnet schools and now small high schools -- that are models. Let's not compromise that history in an effort to support free-market ideology over common sense. Those of us who embrace charter school reform because of its pedagogical promise should encourage our local politicians to focus on what is best for children. That means paying more attention to the data than the rhetoric.

Amy Stuart Wells is a professor of sociology and education at Columbia University's Teachers College and the author of ''Where Charter School Policy Fails.''

Published Friday, Mar. 10, 2006