Well, I know you find this difficult to understand so I’ll try and help.
It’s a law enforcement operation which includes intelligence, they know who the coyotes and cartels are and the routes that they follow.
They also count the number of those they catch.
They base their estimates on those numbers. Typically the BP says they catch about 10% of those attempting to enter illegally.
You do not have to provide ID on the US side when exiting the country into Mexico so they have no way to monitor how many illegals are leaving voluntarily,
Look at this emergency. Look at how its skyrocketing. More proof that much of this is just hysteria from the right. Like the Muslim ban trump called for.
Yeah, this is a lot more complicated than you think.
CBP used to produce estimates but when questioned regarding their methodology they admitted they have basically none. This was during the Obama administration. So they went back and developed some decent methodology. It has a lot in common with epidemiology which is something I know about about. I read the paper and thought it was okay.
No where in there do they estimate they catch 10% of entrants. I’d love to see your source for this.