Here we are again, and I am still harping on the subject of census records. United States Federal Population Census records, to be more precise. The reason I stress these resources is they are easy to get at, and provide more data than you might think.
The first US census was taken in 1790, as was required by Article 1, Section 2 of the very young Constitution. Compare that initial head count, taking up twelve rolls of microfilm, with the 1,854 rolls which hold the 1900 census! More people, more detail.
Take a look at early census records and you will begin to understand some of the limitations of these documents, while other considerations may not be so obvious.
The 1820 census has a list of age ranges for males which seems to be repetitive. It has the logical sequence of 0-10, 10-16 and 16-26, but inserts an extra space for 16-18-year-old free white males (potential soldiers). So don't count someone in that age range twice!
Until 1830, the census office did not supply printed forms, or "schedules." The enumerators simply returned the results on whatever paper that was available (even wallpaper). So we see lists of names (only head of household, mind you) and household members' age ranges in hand-ruled columns, with little scratch marks spaced across the page. And the column designations are not always clear. Deciphering them is sort of like counting threads for cross-stitch patterns. That is why I like to have one of those census fill-in forms handy so I can figure out the headings. (Look for "Supplies, Charts, Forms, etc."on Cyndislist.com)
It was not until 1850 that census-takers got a set of uniform printed instructions, AND that was the year it was determined that every person in the household would be listed, as well as his/her place of birth.
In 1860 post office addresses were added to the top of each page, while 1870 included month of birth if the person was born within the previous 12 months, and the value of the householder's property was listed.
Everyone knows about the increased detail of 1880 records. This census is the first to give the relationship of each person to the head of household, and the place of birth of each person's parents. It also asks whether an individual was married within previous year, and other details.
Most of 1890's census was destroyed by fire so most of us just pass by this year, though fragments do remain.
Some questions were asked on one census but not subsequent ones, so it is necessary to seek out the appropriate decade if, for instance, you need to know whether a member of the household was a naturalized citizen (1910), or the exact year and month of a persons's birth (1900).
Occupations, which were first listed in 1850, can give useful clues, and knowing the street name and number of a dwelling (1880 and on) helps place families in their surroundings.
Of course not all enumerators followed all the directions (such as counting heads as of the precise "census day" specified), people fudged about their ages, neighbors or unattended children were sometimes called upon to help fill in the blanks, and, most maddening of all, sometimes whole pages are missing, faded or blotched. Despite these drawbacks, census records are among our most valuable source of genealogical information.
(And don't forget to look at the neighbors' entries! Extended families often moved in groups from place to another, and a brother-in-law and a raft of cousins may be living just up the street.)
For more detail than you can possibly imagine, as well as copyable census extraction forms, see "The Census Book," by William Dollarhide (HeritageQuest, 2001).