Does anybody know what happens when you do not answer the door for the census bureau in the U.S.?

I sent in my census but 2 weeks after I did I got a postcard saying that they did not receive it.2 weeks after that they came to my house when I wasn't home and left a paper telling me to call and set up an appointment for her to come back. I know that if I do not call her back she will keep coming back several times. But ultimately what happens if I were to not talk to the census lady. I am just curious does anybody know?