The census is a complete joke. In 2000 we got the short questionnaire but that still asks more than the key question so the only answer they got was how many people lived at the house. The other stuff like ages and races (and some of the choices they gave for answers to that I think were just made up, like the multiple subsets of hispanic and I'm not even sure hispanic is a race to begin with) went unanswered. A pair of census "workers" did come to the door but they only wanted to know about the neighbors. I just said I would only speak for myself (and I was at least more polite than the time the UFO cult came to the door).
Since 1790, the census has asked age, sex, and race questions. Before 1870, they also wanted to know how many slaves were in the household.
After 1850, the census had the occupation of at least the head of household.
In 1900, they asked how many years each person had been married, month and year of birth for each person, how many children each married woman had given birth to and how many of those children were still living.
Very useful information for genealogists, although the data isn't made public for at least 70 years (you can see the 1930 census now, and possibly find your parents or grandparents on it.)