North Carolina may be a few years away from using student test scores to measure the performance of teachers, but a new study released this week does just that.

Using North Carolina as their focus, researchers from the University of Washington and the Urban Institute found that teachers who have earned national certification are more effective in raising student achievement than other teachers.

The study analyzed more than 600,000 test scores of Tar Heel students in third through fifth grades from 1996 to 1999. The results of individual students were linked to the teachers who taught them.

The end-of-grade test scores of students whose teachers have been certified by the National Board for Professional Teaching Standards improved an average of 7 percent in reading and math, compared to students whose teachers had tried, but failed, to earn the voluntary credential.

The study also found that nationally certified teachers are more effective with younger students and with those from low-income families. Reading gains were as much as 15 percent greater for low-income students.

"I was shocked to find anything," said Dan Goldhaber, a research associate professor at the University of Washington and co-author of the report. "I was surprised that board certification would make a difference."

To earn a certificate, a teacher must demonstrate strong teaching practices and solid knowledge of the content area and methods. About half of the teachers who apply for the credential earn it.

North Carolina teachers who are certified are rewarded with a 12 percent salary supplement, one of the most generous incentives paid by any state. The extra pay is one reason North Carolina claims more of the 32,000 certified teachers than any other state.

Nationwide, more than $350 million has been spent since the mid-1990s on the national certification program.

The new study is being called the first large-scale analysis of the classroom performance of teachers who have national certification.

North Carolina this year is spending about $38 million on national certification. Most of that -- about $32 million -- is for salary supplements, which amount to an average of about $5,000 a year for the 6,000 teachers now holding the credential.

The state pays the $2,300 application fee required for teachers seeking the certificate.

A question left unanswered, Goldhaber said, is the cost effectiveness of the certification.