A new federal initiative that could provide millions of students with a free lunch might have an unexpected cost for researchers and state educational agencies.

Starting in July, many high-poverty schools where at least 40 percent of students qualify for a free or reduced lunch could begin to offer that free lunch to every student — regardless of income — under the new community eligibility provision of the National School Lunch Program.

In the process, however, expanding the program presents challenges for researchers and educators that have for decades used participation in it as a proxy for poverty in tracking student performance.

“It’s obviously good for kids, but from a pure data perspective it provides some weaknesses,” said Brandon LeBeau, an assistant professor at the University of Iowa’s College of Education who has studied the use of free lunch eligibility in education research.

Two analyses found that school lunch data has been used in about 1 in 5 studies looking at academic achievement conducted by education researchers; that doesn’t take into account its role in work by psychologists, sociologists, economists and researchers in a host of other disciplines.

The data has been used in studies looking at best teaching practices, school discipline and whether playing an instrument improves academic performance. It was a measure of poverty for a study on why kids start smoking, used to differentiate swimming ability among minority students, and a measure of socioeconomic status in at least one article in the Journal of Happiness Studies.

Who gets to participate in the program can have policy implications as well.

Matt Cohen, chief research officer at the Ohio Department of Education, is chairing a working group organized by the National Center for Education Statistics (NCES) to find alternatives to participation in the school lunch program for measuring students’ socioeconomic status.

The free lunch expansion “muddies the water,” he said, making it harder to isolate the performance of students from truly disadvantaged backgrounds. Scores on the National Assessment of Educational Progress (NAEP), for example, are broken out by eligibility for the school lunch program. In 2013, fourth-grade students eligible for the lunch program scored about 10 percent lower on average than their non-lunch-eligible peers in math and about 14 percent lower on average for reading. Lumping both groups together in high-poverty schools could erase those differences and mask problems, or obscure progress.

The expansion affects funding, too, as state funding formulas typically award extra funding for schools based on the number of students they have in the program. In Ohio, for example, schools are annually given additional funding — $272 this year — for each “economically disadvantaged” student, including students eligible for free or reduced lunch.

“When money’s involved, people are intensely interested,” Cohen said.

The school lunch program was first created in 1946 in response to concerns about the number of men denied entry to the Army in World War II because of undernourishment. Eligibility for the program is primarily income-based. Students whose family income is at or below the federal poverty level — $23,850 for a family of four this year — are currently eligible for a free lunch, and students are eligible for a reduced-price lunch if their family’s income is up to 185 percent of the poverty level.

Although schools have reported participation levels in the program for decades, the connection to school performance was given extra attention with the 2001 federal No Child Left Behind law, which required states to report the academic performance of various student subgroups, including economically disadvantaged students. The law didn’t specify how states had to report that data, but in practice most states have used the performance of students eligible for the school lunch program to meet those requirements.

But even before the expansion, LeBeau and others have suggested that lunch eligibility was a flawed measure of student poverty. “I think a lot of researchers don’t really think about it,” LeBeau said.

Participation in the program requires many parents and students to enroll — meaning that some students who are eligible don’t participate (and vice versa). Under-enrollment is particularly acute at the high school level, where some eligible students elect not to enroll because of a perceived stigma associated with participation in the program.

Cohen suggested that in Ohio and other states, agencies could use the lists of students who participate in the federal food stamps program or Temporary Assistance for Needy Families program, commonly referred to as welfare, as an alternative means of tracking the academic performance of lower-income students or awarding additional school funding. They already have access to the lists and are encouraged to refer to them when pre-qualifying students for the lunch program.

Chuck Cowan, a former chief statistician for NCES, said researchers and education agencies should go beyond income to develop a better understanding of a student’s socioeconomic status. The first challenge, though, is figuring out how to define socioeconomic status in the first place.

“Sociologists tend to focus more on parents’ occupation, and economists would focus more on income, and psychologists are probably split equally,” he said.

Cowan co-wrote a 2012 report commissioned by the governing board of NAEP on improving its measurement of socioeconomic status. The report suggested that NAEP incorporate, at minimum, the occupation and educational levels of parents to develop a socioeconomic variable that could be linked to student performance on the national standardized test. Already, eighth- and 12th-graders are asked about their parents’ educational background, while all students are asked about access to technology and other aspects of their home life. Down the road, the report suggests that neighborhood factors — such as unemployment figures or access to a public library — could be incorporated to give a more nuanced understanding of which factors contribute the most to academic performance.

In the absence of such a variable, Cowan said, he’s not sure what he would recommend researchers use.

“I think the national school lunch program was a good proxy back in the day,” he said. “Now what it measures is something different.”