I am a former employee of the CB. I do NOT speak for the Census Bureau.Your question is somewhat circular. The US government does not know if everyone in the USA is counted. That is the reason for the census in the first place!!!!The objective and goal of the census is to count everyone. But no census has ever been 100% accurate, and none ever will. The census is a GOVERNMENT operation, and therefore prone to government-think, and government f***-ups. Trust me on this.The fact is: The census is the largest government operation, which is conducted primarily by amateurs (college kids, housewives, retired people, etc). It always comes in on time, and under budget. The people who work on the census, are not contaminated by lifetime federal employment.The Bureau purchases junk mail lists from commercial vendors. Then it sends a census form to every person on the junk mail list. Some of the recipients may have died or moved, or the address is no longer valid. In 1980, I did not have a residence address, I got my mail in a PO Box. I never received a form in the mail. I was not counted.There are various ways to get the forms to the people in the USA. In high-rise housing apartment buildings, the bureau just gets a wheelbarrow full of forms, and dumps them in the lobby, with a poster telling people to pick up a form, fill it out, and mail it in. ( compliance in federally-operated housing projects is low).The bureau tracks every mailed form. If a form is not returned, then a check is made. If the address is valid, and there are persons domiciled at that address, then an u201cenumeratoru201d is sent there is person, to collect the information.ALL people who are in the USA on April 1, of every year that ends in a u201c0u201d, are required to be counted. This includes citizens, aliens, (legal and illegal), and nationals (residents of the US territories, who are not citizens). ALL people , including people in hospitals, jails, retirement homes, etc.When a person gets a form, they are required to fill the form out completely and honestly, and return it promptly. Deliberately withholding information, or falsifying information, may result in prosecution and/or imprisonment. (prosecutions are rare).It is the DUTY of every person, citizen, alien, and national, to complete the form and return it promptly. The US relies on peopleu2019s patriotism, and the threat of federal imprisonment, to ensure compliance.