As the District works to confront rampant truancy in city schools, national standardized test data show that D.C. students are absent from school more than the national average and more than almost all other large U.S. cities.

Data from the 2013 National Assessment of Educational Progress, a test administered every two years to fourth- and eighth-graders across the country, also show that students who miss the most school tend to score lower in math and reading than peers who attend more regularly.

“We clearly have a long way to go on truancy and our truancy rates are unacceptable,” said Deputy Mayor for Education Abigail Smith, who fielded many questions about truancy Friday at a D.C. Council hearing. “But our truancy rates are going in the right direction. They are going down.”

Students who take the NAEP exam are asked how many days of school they missed in the month before the test. In 2013, 28 percent of eighth-grade students in the District’s traditional school system said they had missed three days or more, a rate equivalent to about five weeks of missed instruction during the course of a year.

Such chronically absent students scored 15 points lower in math than their counterparts who missed no days of school in the month before the exam was administered. Fifteen points is roughly equivalent to about a year and a half of instruction, according to Alan Ginsburg, a retired U.S. Education Department official who co-authored a 2012 study of the connection between absenteeism and NAEP performance.

“At lot of things have improved in D.C.,” Ginsburg said. “This is an area where they still can improve.”

Only two other cities had a higher proportion of eighth-grade test-takers in math who missed as much school. In Detroit, 33 percent of students missed three days or more; in Cleveland, it was 30 percent. Nationwide, 20 percent of eighth-grade students said they missed three days or more.

D.C. Public Schools eighth-graders who took the NAEP reading exam also had among the highest absenteeism rates in the country, as did DCPS fourth-graders. But in fourth-grade reading, there was a whopping 22-point difference — or more than two years’ instruction — between students who did not miss any school in the month before the exam and those who missed three days or more.

The correlation between absenteeism and performance is not surprising. Teachers cannot teach students who are not in class. Despite the clear connection between attendance and performance, NAEP data suggest that nationwide absenteeism rates have not budged over the past two decades, according to Ginsburg’s analysis.

Ginsburg said the city should publish more attendance data. The truancy data that schools currently release should be expanded to include data on the proportion of students at each school who are missing a lot of school for any reason, he said.

That recommendation is echoed by attendance advocates across the country who say that focusing on truancy alone misses the broader problem of chronic absenteeism, which includes excused absences.

Ginsburg, who now works as a part-time teacher at a charter school in the District, said that students in his classroom who accumulate absences tend to fall behind and stay behind. “These kids may have a note from their parent, but they’re absent a lot,” he said. “If they’re absent a lot, it just kind of feeds on itself.”