MARIETTA — According to a new calculation method mandated by the U.S. Department of Education, both Cobb and Marietta school districts saw drastic drops in graduation rates.

Under the new method, Cobb’s graduation rate for its 16 high schools went from 84.7 percent to 73.3 percent, dropping 11.4 percentage points, and Marietta City School’s graduation rate, which includes figures from Marietta High School and the two residential treatments centers, dropped from 85.8 percent to 56.0 percent, or 29.8 percentage points.

The state’s graduation rate dropped from 80.9 percent to 67.4 percent, or 13.5 percentage points.

“We anticipated about a 20 percent drop, so there was a definite surprise when we saw the 27 percent drop,” said Dr. Debra McCracken, Marietta City Schools’ assistant superintendent for curriculum and instruction.

Cobb’s Chief Academic Officer Dr. Judi Jones was unavailable for comment before press time on Tuesday.

The new method calculates the number of students who enter and exit a school between their ninth and 12th grade years. Previously, schools only considered students’ senior year.

If a transfer student is not properly documented by both the school they leave and the new school, their former school will have to count the student as a dropout, which hurts the school’s graduation rate.

McCracken said the city district is “not pleased with the results.”

“We presently have three research studies that we are engaged in that all examine failure rates,” she said. “Know that because we are actively engaged in the research studies, we are going to become very aggressive in finding solutions.”

McCracken said the district will aim to meet or beat the national graduation rate, which has not yet been determined by the U.S. Department of Education.

“We have a high-quality staff, we have strong leadership in the high school, and rest assured, this is just a new method of calculating the graduation rate and our program will only get better,” she said.

While Marietta Board of Education Chairwoman Jill Mutimer said there is “no perfect answer to measure something,” she said the graduation rate is “bad. B.A.D. But it’s not going to be for long.”

“What I want to know is how much of the 40 percent is about what. How much of the 40 percent is related to immigrant students? How much is related to special ed?” she said. “Maybe under the old method we didn’t do as good a job tracking where our students went because it didn’t matter. Maybe now we need to do a better job of that.”

Marietta school board member Irene Berens said state superintendent John Barge had prepared her for bad news.

“It is difficult to track these students, and I’ve always been concerned about our transiency rate, and this points out that we really need to focus on that,” she said.

Neighboring school districts also saw large drops: Gwinnett’s rate dropped from 83.7 percent to 67.5 percent, Cherokee went from 82.1 percent to 74.8 percent, Fulton went from 85.9 percent to 70.1 percent and Atlanta went from 68.2 percent to 51.9 percent.