A while ago I was expounding on the various aspects of the United States Federal Population Census records, and had gotten as far as the (mostly) doomed 1890 enumeration. So let's carry on.
The 1900 census has more questions than any other, and so is a great source of information for the genealogist working in that time period.. The form has spaces for indicating each person's month and year of birth – so those dates can be narrowed considerably. There is also a column for the number of children borne by the women, and number still living. "Years married" is also asked. All these help one draw, confirm or discard conclusions about a family and its individual members.
Another helpful section covers citizenship. If you are looking for immigration information, be sure to check this. Individuals are supposed to indicate their year of immigration, number of years in the U.S., and whether naturalized.
The 1910 census does not ask for month or year of birth, just the person's age (which, as you know by now, can vary considerably from one census to the next, often depending on who is giving the information). It does, however, expand on occupation, asking not only for "trade or profession" but also "type of business," and whether the individual is an employee, employer, or "works on own account." It also has a column to indicate whether the person is a veteran, an often overlooked item which could lead to further information sources.
1920 is much the same, though it has space to indicate not just street name, as in the previous census, but the actual address. In addition to birthplaces of the individuals and each of their parents, it has space to indicate "mother tongue." Citizenship, too, is addressed again.
There is an interesting story behind the original indexing of some of these records. William Dollarhide, in his "The Census Book" (Heritage Quest, 2001) discusses the original intent of the Soundex indexes, which were created for 1880 through 1920. They established a code for surnames in an attempt to bring together names which were phonetically similar (like Cain, Cane, Kain, Keen, Koeghn, etc.). The intent was to enable the Social Security system, which would begin making payments in 1937, to locate individuals in the census records and thus prove potential recipients' ages. Every-name indexes were done in this Soundex code for the 1900, 1910 and 1920 censuses, while the 1800 census was indexed only for families with children aged ten years or under.
In pre-computer days, we genealogists used to have to determine the code for our surname of interest (initial letter plus three numbers), find and search the corresponding microfilm, then use that information to find the census record itself.
There are other census records besides those designated "population." Another time.