Summary files/counts

For the Decennial Censuses, the Census Bureau has for many years conducted effectively two surveys. A 100 percent survey in which every single person in the country is supposed to be accounted for and a sample survey conducted for a random selection of households. For Census 2000, for example, this sample survey was done for one in every six households.

One of the reasons for the Census Bureau to do so is that the sample survey has many more questions than the 100 percent count. Moreover, it would simply be too expensive to conduct this survey for every household. It is also regarded as mostly unnecessary as the sample in the sample survey is so large it almost amounts to the 100 percent count. 

The sample survey, however, contains a much more detailed profile of the U.S. than the 100 percent survey which only asks questions regarding sex, age, and race.
These surveys are then processed into summary counts or files. Let's take a look at the Census 2000 and 1990 and their summary files.

Census 2000

Census 2000 consists of four Summary Files (SF).

SF 1

SF 2

SF 3

SF 4

If the data you are looking for is located in the first two summary files (SF1/2), you should use those numbers, because they are more “accurate”, otherwise you have to use SF 3/4.

What are SF2 and SF4 used for?

Most people have never used these two Summary Files (SF) because they are somewhat more complicated. They contain the same data tables as SF1/3 respectively, but the tables are iterated for 336 race and ancestry categories. For example, if you are interested in Household Income for Japanese people, you would use SF 4. SF 2/4 provide data down to Census Tract level.

Census 1990

For Census 1990 the Census Bureau has produced four Summary Tape Files (STF). They are equivalent to the Summary Files in Census 2000. Summary Tape File 1/2 (STF 1/2) contains 100% data on age, race, sex, marital status, Hispanic origin, household type and relationship, occupancy/vacancy status, tenure, units in structure, contract rent, meals included in rent, value, and number of rooms. Summary Tape File 3/4 (STF 3/4) contains sample data weighted to represent total population. Again as in Census 2000, STF 2/4 data tables are iterated by detailed race and ancestry categories.

What happened to summary files in Census 2010?

For Census 2010, the questions were divided between short and long-form. The short form was conducted on a 100% sample of the U.S. population, while the long-form was done on a 1-in-6 sample of the population and consisted of a much wider selection of questions. After the 2000 Census, this long-form became known as the American Community Survey or ACS.