Georgia has one of the lowest high school graduation rates in the country, according to a national study released Wednesday.

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About 58 percent of Georgia students graduated on time, compared to the national average of about 71 percent, according to the report. The data comes from the 2004-05 school year, the most recent year for which national comparison data was available. Only four states and the District of Columbia performed worse.

Among the nation's 50 largest school districts, two metro Atlanta systems ranked near or just above the national average while two were below. Cobb was the highest at 71.6 percent, followed by Fulton at 71 percent. Gwinnett was below the national average at 68.4 percent, as was DeKalb at 58.9 percent.

The report, "Diplomas Count 2008," was released by Editorial Projects in Education, a nonprofit that publishes Education Week, an education journal.

The study's graduation rate for Georgia is lower than the 69.4 percent state Department of Education reported for 2005. Both groups used different formulas for their calculations.

"No matter how you calculate the graduation rate in Georgia it is too low and the focus of everything we're doing is to raise that graduation rate," said Dana Tofig, spokesman for the Georgia Department of Education. "One thing that is good is that the Education Week report is showing growth in our graduation rate and our data is showing growth. No matter how you calculate it, Georgia is going up."

The study says Georgia's graduation rates has increased modestly since 2003 when it was 56 percent, using their own calculations.

There's no uniform way to calculate graduation rates.

The report's researchers used a formula to estimate the likelihood students will complete high school on time and with a regular diploma. The researchers tracked the four steps students must take to earn a diploma — three grade-to-grade promotions and grade 12 to graduation, according to the report.

Georgia uses one of three formulas approved by the federal education department, taking into account the number of students who drop out.

Dropouts are hard to track and some researchers say states tend to undercount these students, which inflates graduation rates.

The complete report can be found at: www.edweek.org/ew/toc/2008/06/05/index.html.