Researchers, grant-makers and policymakers have long relied on enrollment numbers for the federally subsidized Free and Reduced-Price Lunch program. They use those numbers as a handy proxy for measuring how many students are struggling economically. The paperwork that families submit to show their income becomes the basis of billions in federal funds.

To be eligible for these programs, a family must earn no more than 85 percent above the poverty line. Just over half of public school students fit that description.

At least, that’s today’s figure. As Jill Barshay reports for the Hechinger Report, things are about to get more vague.