Do charter schools work? Figuring out the answer should be easy. Yet a sequence of reports last month left the impression that it is more confusing than it needs to be. So here's a "how-to" guide to interpreting December's dueling charter-school studies.

One study, issued by the Department of Education's National Center for Education Statistics (NCES), reported that charter-school students lag behind those in traditional public schools. Another, released a day earlier by Harvard University economist Caroline Hoxby, said charter schools are doing better by America's children than traditional schools.

How do we account for the difference?

NCES, to its credit, points out that its study must be read closely and critically. For instance, the better performances by students in reading and math in public schools disappears when the results are broken down by race, although they are statistically significant among students eligible for free and reduced lunches and those in inner cities.

The NCES study also points out "while charter schools are similar to other public schools in many respects, they differ in several important ways, including the makeup of the student population and their location."

Plus, parents choose charter schools for their children. Children are not assigned to them, in contrast to the way things operate in traditional public schools. And often, they're chosen precisely because traditional public schools have failed the child. So a comparison of all students from traditional public schools versus all those from charters doesn't accurately reflect whether charters do their job.

What does work is the approach Hoxby took. But don't just take our word for it. As NCES Associate Commissioner for Assessment Peggy Carr remarked at the NCES study release, matching, the methodology Hoxby used, "is a more superior design."

To do this, Hoxby looked at virtually all students who attend elementary charter schools and compared them to students who attend the nearest equivalent public school, yielding "matched pairs" of student bodies. She found charter students' proficiency in reading to be 5.2 percent higher and in math 3.2 percent higher than those in their matched public schools.

Further, Hoxby's study shows that charter-school students' gains in academic achievement, relative to their public-school counterparts, tend to increase for more established charter schools. In other words, charter schools are pulling away from public schools in terms of performance.

For example, children at charter schools that have been operating for one to four years are 2.5 percent more proficient in reading than students in the nearest comparable public school. That proficiency advantage increases to 5.2 percent for charter schools open five to eight years and to 10.1 percent for charter schools that operate nine to 11 years.

When comparing charters with established public schools, it is only reasonable to evaluate charter schools after several years of operation. Many charter schools included in the NCES study, however, had been open for only a year or two.

Many charter schools in minority and high-poverty areas show even greater achievement gains, relative to their public-school counterparts, than charter schools in general. For example, students at charter schools in Hispanic areas have a 7.6 percent advantage in reading. Charters in African-American areas have a 4.5 percent advantage, compared to a 4.2 percent advantage for students in charter schools in other areas. Similarly, students at charters in high-poverty areas have a reading proficiency advantage of 6.5 percent, compared with a 2.6 percent advantage for students in other charter schools.

Charter schools are currently educating more than one million children nationwide. Given Hoxby's results, let's hope for the continued spread of charter schools in 2005.

Jennifer A. Marshall is director of domestic policy studies, and Kirk A. Johnson, Ph.D., is a senior policy analyst in the Center for Data Analysis at The Heritage Foundation (heritage.org).