Prisons are considered "group quarters" and don't get counted on that form; either the prison fills out a form or a census taker goes to the prison and counts everyone, depending. (Guess what I've been doing for the last month! And I'm most put out that all the hard work we put in is now interrupted at the critical point; this is the time when pretty much my entire job should have been making sure we were contributing as much as we possibly could to the accuracy of the town's count.)