The gap in NAEP test scores between students who reported excessive absences and those who didn’t exceeded a grade level of knowledge in both reading and math.

While the correlation between missed school and lower achievement is well established, researchers said they were struck by how the trend held in every state, grade level and student category.

The data help estimate chronic absenteeism — which Attendance Works defines as missing 10 percent of the school year, regardless of the reason, though state definitions vary.

Students who are chronically absent are more likely to struggle academically by third grade and are at increased risk to drop out of high school.

Many states, including Montana, measure truancy and average daily attendance but not chronic absenteeism.

Montana uses average daily attendance as one part of its annual No Child Left Behind report card. School statewide reported an average daily attendance of 94 percent, well above the target of 80 percent.

Researchers said states would be better off tracking chronic absenteeism data, which tracks individual students, as well. That information could be used to identify struggling schools or keep tabs on students who move between districts.