Politicians on both sides say that the Obama administration's executive actions on immigration from November 2014 will protect millions of unauthorized immigrants from deportation, often citing a number in the range of 5 million. But that's probably not true — because no one's automatically being granted protection from deportation. Immigrant parents of US citizens and green-card holders who qualify for the program that President Obama announced in November have to apply in order to actually receive deferred action (a temporary grant of protection) and a work permit. So to get an accurate count you need to estimate: how many immigrants who are eligible will actually sign up?

The Congressional Budget Office tossed out one answer to this question on January 15. In a report about the possible budgetary impacts of the president's executive actions, they estimated that 2 to 2.5 million unauthorized immigrants will have deferred action by 2017. But the CBO's estimate isn't authoritative — or even, necessarily, particularly well-grounded. And before anyone really knows how many people will apply for relief from deportation, there are big questions that still have to be answered about what the requirements are going to be — and how effectively activists and community groups will be able to reach unauthorized immigrants and help them apply.

Why it's really hard to estimate how many people will apply for relief

The CBO didn't explain how it arrived at its estimates, to the frustration of immigration policy wonks. But Marshall Fitz of the Center for American Progress recommends that, instead of trying to reverse-engineer the CBO's math, "we should just treat this estimate with a grain of salt." Here's why.

No one can even say for sure how many unauthorized immigrants there are in the United States or what their demographics are. So as Fitz puts it, simply "estimating how many people might be eligible" for deferred action under the Obama administration's executive actions "requires one big set of assumptions." Several think tanks, not to mention the federal agency that will be processing immigrants' applications, have tried to make some educated guesses about how many people are eligible, but they haven't gone so far as to estimate how many people will actually apply.

The CBO, meanwhile, doesn't have the same level of expertise in immigration policy those think tanks do. Predicting immigration policy outcomes isn't the CBO's job — though in this case, it's something the agency has to do on the way to its actual job of estimating how much the policy changes might cost. So the CBO is taking the think tanks' set of assumptions about eligibility and stacking another set of (unexplained) assumptions on top of it.

Simply extrapolating from Obama's first immigration program won't work

The CBO and others aren't flying completely blind. They can look at the existing Deferred Action for Childhood Arrivals program, which started in 2012 to give relief from deportation to young unauthorized immigrants who'd be eligible for the DREAM Act, and work from there. (About half of the people who are eligible for DACA applied for it in the first two years.)

But there are some big differences between the people who were eligible for DACA, and the people who'll be eligible for the new program. Sociologists of immigration (taking a cue from a term invented in the 1920s) say that immigrants who are eligible for DACA are members of the "1.5 generation" of immigrants. Because they arrived in the US as children or teenagers and have gone to school here, they have just as much in common with people who've been born in the US to immigrant parents as they do with people who came to the US as adults. They're almost guaranteed to be fluent in English and to be socially integrated in the US. Both of those factors make it more likely that they'd hear about the deferred-action program and figure out how to sign up.

Furthermore, young unauthorized immigrants had been organizing politically for years before the DACA program arose — so there was a ready-made pool of easily reached, motivated immigrants who'd be able to sign up for the program themselves and then spread the word about it to their friends, classmates, and colleagues. That level of organizing hasn't happened among the older generation of immigrants. "DREAMers" have been coming out as unauthorized for years — older immigrant workers have only just begun to follow in their footsteps.

On the other hand, it's possible that it might be easier for parents of US citizens to get their applications together than it was for young students. For one thing, as CAP's Fitz points out, an adult will have an easier time pulling together the money for the application fee (even if she has to take out a loan to do it) than a 20-year-old will. For another, pulling together the documentation for the application might be easier. Many immigrant parents have an easy way of showing they've been in the US for more than five years: their children's birth certificates.

The big unanswered questions about how the new program will work

In addition to all of those factors, though, there are still things that we just don't know about how the new deferred action program is going to work. And those things are going to have a tremendous impact on how many people sign up.

How much will it cost to apply? Applying for the existing Deferred Action for Childhood Arrivals program costs $465 — and renewing it (which people currently have to do every two years, though new applicants will get three years of protection starting this spring) costs the same amount. That's a lot of money for people who often don't have much to begin with, and studies of immigrants who'd be eligible for DACA but haven't yet applied show the biggest problem is cost. Even if it's easier for adults to pull that money together, there will be many more applicants, and many more people applying quickly, if the application fee is lower.

How strict will documentation requirements be? After years of trying to hide from government authorities, it's often difficult for unauthorized migrants to produce documentation of their presence and employment in the US. The more kinds of documentation the government accepts to prove an immigrant is eligible — like affidavits from employers — the more immigrants are going to be able to apply.

How well will activists, lawyers, and community organizations be equipped to educate immigrants and help with applications? One of the lessons of the DACA program was that misinformation runs rampant. That makes it a lot harder to get an immigrant to put a reliable application together without throwing away money to a shady notario — a fraudster who claims to be an immigration law professional but isn't actually a lawyer. And the government simply doesn't have the resources to educate immigrants about what they need to do for the application. Nor does it have the resources to return half of the applications it gets and ask people to try again — people whose applications aren't complete probably aren't going to get approved.

So community groups need to take the lead on educating immigrants about the program, and helping them put together a near-perfect application. That takes a lot of money, and a lot of organizing — and while many groups have some experience helping out with DACA, this is a much bigger program. It's not clear just how well equipped civil society is going to be.