In Part 2 we saw how to break down each year’s unauthorized arrivals into the overstays and the migrants that entered successfully through borders, almost always the southwestern border. But there are two groups of border crossers who join the unauthorized population: the migrants who entered without being apprehended, and the migrants who were apprehended but were allowed to remain—that is, were admitted temporarily to pursue claims for a legal status. (These migrants are still subject to deportation if they fail to win their cases.)
There are three
different ways of estimating the number of migrants who entered the U.S.
without being apprehended—the “gotaways,” in the Border Patrol’s actual official
terminology..
The ACS-Based
Apprehension Rate
The first method relies on estimates that demographers make for each year’s number of new arrivals in the undocumented population; they base these on the Census Bureau’s American Community Survey (ACS; see Part 2). New arrivals consist of migrants who overstayed their visas, migrants who were apprehended at the border but were allowed to enter, and migrants who evaded apprehension.
For the number of new arrivals in 2016, for example, we can use the 2019 CMS report or Robert Warren’s 2018 report
in combination with the unpublished CMS data we cited in Part 2. The 2019
report estimates 515,000 total arrivals to the undocumented population in 2016,
while the unpublished CMS data puts the number at 623,000. From these estimates
we can subtract the estimated overstays, 320,000 in the 2019 report and 306,000
in the 2018 report, to establish how many entered at a land border.
Next, we need to estimate
the number of migrants that agents apprehended but allowed to enter the
country. (As we noted in Part 1, these are mostly asylum seekers who are
admitted to the country pursuant to two laws
that establish procedures to adjudicate their claims. Many come as family units
or unaccompanied minors, and this category started increasing significantly in
2014; for example, 77,674 family unit members and 59,692 unaccompanied minors
were apprehended at the southwestern border in 2016.)
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| Source: Congressional Research Service |
The
Border Patrol reported 408,870 apprehensions at the southwestern border in fiscal 2016, and 245,400
border crossers were removed that year, almost all from that same border. That
leaves 163,470 migrants, mostly asylum seekers and unaccompanied minors, who
were admitted to the country after being apprehended.
Once we subtract the overstays and the admitted migrants from the total arrivals, we have a rough idea of how many migrants entered without being apprehended, and we can use this number to estimate the 2016 apprehension rate—that is, the percent of border crossers that DHS succeeded in apprehending.
Using the 2019 CMS report’s
estimate of 515,000 total undocumented arrivals and subtract the 320,000
overstays and 163,470 admissions, we estimate that about 31,570 migrants evaded
apprehension at the southwestern border in 2016. By adding this 31,570 to the
408,870 total apprehensions, we arrive at some 440,440 unauthorized crossings at
that border. This gives us an apprehension rate of 93 percent.
If instead we use 2016 arrival number from the unpublished CMS estimates—623,000—and the overstay number from Warren’s 2018 report—306,000—then 317,000 migrants arrived by crossing the border without authorization. Subtracting the 163,470 admissions, we get 153,530 migrants who evaded apprehension. Adding this to total apprehension gives us an estimated 562,400 border crossers, with nearly 73 percent of them being apprehended.
Model-Based vs.
Observational Apprehension Rates
The US government has a
different way to estimate the number of unauthorized migrants who cross the
border without being apprehended. In fact, it has two.
The government’s two methods
are explained in an August 2020 DHS report
on border metrics. One is a model-based apprehension rate that extrapolates
from a survey the Colegio de la Frontera Norte takes of migrants who have recently attempted to
cross the border. The other is an observational apprehension rate. This is
extrapolated from Border Patrol observations of people crossing successfully
and from evidence like footprints.
DHS reports on the model-based apprehension rate go back to fiscal 2000. This shows a dramatic increase, from as low as 32.5 percent in 2003 to a high of 83.9 percent in 2016, with a tendency to rise throughout the period. DHS didn’t start calculating the observational rate until 2006; it too shows a tendency to rise, but not nearly as much as the model-based rate. The lowest rate is 63.5 percent for 2006, while the highest is 79.4 in 2011 both and 2016.
 |
| Source: American Immigration Council 2021 Report |
The model-based rates seem much
too high for the early period. For example, the Border Patrol reported 1,643,679 apprehensions for 2000. The model-based apprehension rate for that
year was 42.5 percent, so the number of successful border crossers would have
reached some 2,224,000. But Warren and Warren’s estimates based on the ACS only
show a total of 1,389,322 undocumented immigrants entering the country that
year—and this includes both border crossers and overstays.
The observational rate seems to give a more convincing result, and the government’s two different apprehension rates have been close to each other since 2015, which suggests that that they may both be fairly reliable now. For example, the observational rate for 2016 was 79.4 percent, while that year’s model-based rate was 83.9 percent. Applying the observational rate to the 408,870 reported apprehensions, we would estimate that a total of 106,000 migrants crossed the border that year without being apprehended. The model-based rate would yield about 78,000 crossers who weren’t apprehended.
These
rates from DHS are about halfway between the rates we calculated from the two
different ACS-based estimates. This suggests that the actual rate now is
somewhere around 80 percent.
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