According to the rules, those "incomplete answers" qualify you for the $100 fine bracket. Knowingly giving false answers (like, "I'm not sure", when you are sure) moves you into the $500 fine bracket. A business "agent" (?) is subject to a $10,000 fine for providing false information.

Again, the national census is out of date. It was OK when it took 2 days to ride/walk from one side of the county to the other, but not today, not in 2015.

That "confidential" information won't remain confidential forever either. I'm certain that are some folks that participated in the 1940 census that are still alive today. Here's link to their info, though I admit it is interesting to read now:

http://1940census.archives.gov/