Census is usually done every 10 years (I think).

They send out vague questionnaires to every residence, asking how many people live there. The race, age, if you own your house or rent, etc. I've attached the valid file, which is what yours sounds like to me.

In my opinion, the census Q's may seem strange but they are informative statistically. Rental areas typically need more police activity, to keep the area safe. The same general rule also applies to large minority housing areas, low household income levels, as well as areas with low education levels. It's all statistics, but it does get taken into account for city planning and such.

http://www.census.gov/acs/www/Downlo...13/Guide13.pdf

There's a copy of the census right above to refer to. Hope that I helped answer your Q's!