system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If they're undocumented immigrants, how does the US know that there are 11 million of them?
gpt
The US doesn't know; but it estimates. 

There are many different estimates and approximations. The most cited number comes from census data. The census does not ask if someone is legally in the country or not, but it does ask about place of birth. The government does know how many foreigners are permanent residents and how many have become citizens, so subtracting the legal immigrants from the foreign-born census respondents yields the number of illegal immigrants.

This number is an estimate, and far from perfect. Not everybody responds to census, for instance, and many illegal immigrants specifically don't respond. So the number is refined by other metrics, is compared with other things known about people. Even if they are illegally in the country, they still need to pay rent, drive, work, do other things. Combining all this information with statistical analysis generates the estimate.