“One person, one vote” is a deceptively simple promise, but a Texas woman wants to clarify which persons count. On Tuesday the U.S. Supreme Court agreed to hear Evenwel v. Abbott, a suit that challenges exactly who should be counted as a person when states draw their district boundaries in pursuit of proportional representation.

The plaintiffs are challenging the usual method (counting total number of people living in a district) and are asking that states use the total number of eligible voters instead. The trouble is, we don’t have robust statistics on the number of eligible voters. If the Supreme Court were to set new standards for districting, we would need to overhaul the nation’s statistics and surveys.

Electoral College votes and congressional district boundaries are determined based on Census figures for population. Those numbers include noncitizens, prisoners, felons, children and other people barred from voting. To get from the Census numbers to an eligible voter estimate, a district-drawer would need to make a lot of slightly suspect adjustments.

From the Census numbers alone, it’s possible to calculate the voting-age population (VAP) as a crude approximation of the total number of eligible voters. VAP is just the Census tally, minus everyone under the age of 18. This adjustment wouldn’t placate the plaintiffs of Evenwel v. Abbott, who are more concerned about noncitizens who, provided they’re of age, would still be included in VAP figures. (Sue Evenwel is the plaintiff, and the case is filed against Texas Gov. Greg Abbott.)

But the Census doesn’t ask respondents about their immigration status. In order to get an accurate count of the population, the Census form has to be short and relatively uncontroversial. This encourages people to fill it out without too much inconvenience or fear of the consequences.

To get a sense of the geographic distribution of citizens, the Census numbers have to be adjusted again using statistics from a different Census Bureau project, the American Community Survey (ACS), which does ask about citizenship. The ACS’s citizenship questions make it possible to change the Census’s count of the voting-age population into an estimate of the voting age citizen population.

Unlike the Census, the ACS is not a complete count of the country. The goal of the Census is to reach everyone; the ACS uses a representative sample of the nation. Sampling can be more accurate than counting, but efforts to use sampling to allow the Census to account for people it misses were blocked by the Supreme Court in 1999, on the grounds that the Constitution requires a traditional, full-count Census.

Relying on the ACS for this adjustment may be harder to do in the future because the ACS has struggled to get support from Congress. In 2012, the House of Representatives passed a bill that would have eliminated the ACS. Although that proposal did not become law, ACS funding has been cut to the point where the Census Bureau can no longer provide three-year rolling averages of its figures. Provided the ACS continues to exist, it’s the best option available for calculating the citizen voting-age population. But that’s still not enough to estimate how many eligible voters live in a district.

At the end of 2013, the Bureau of Justice Statistics estimated that 2,220,300 people were incarcerated in the United States. Nearly all of these prisoners are barred from voting. Every state but Vermont and Maine denies voting rights to people in jail. So one more adjustment needs to be added to the eligible voter calculation.

Districting based on total population, rather than eligible voters, usually favors urban areas, but prisoners are an exception to this tendency. Prisoners are sometimes incarcerated far from where they live, usually in rural areas, boosting the electoral clout of a district in which they have never been eligible to vote. The Prison Policy Initiative has identified 21 counties in the United States where at least 20 percent of residents are prisoners.

There’s still one more big adjustment to make. People who have been convicted of felonies are frequently ineligible to vote, even after they’ve been released from jail, but the restrictions vary by state. Some states strip felons of their votes permanently, others have a waiting period, and some require felons to apply for restoration of their rights on a case-by-case basis. This hodgepodge of restrictions makes it hard to adjust the numbers, especially because states may not release detailed data on where these former felons live now.

In a 2001 paper in the American Political Science Review, Michael McDonald and Samuel Popkin suggested two other possible factors to consider when estimating the total number of eligible voters. First, some states have residency requirements that might make people who have recently moved ineligible. Second, citizens can lose their voting rights if they are ruled mentally incompetent. McDonald and Popkin thought both of these factors were more trouble than they were worth to model: The researchers estimated that only 1 percent of the voting-age population failed to meet residency requirements and that only 0.1 percent were mentally incompetent to vote.

If the Supreme Court requires eligible voter estimates for districting, they may need to rule on whether McDonald and Popkin were right to exclude those groups — and on a host of other methodological questions — as the states scramble to commission constitutionally compatible surveys.