Census Quirks

Geography

• Political geographic boundaries are too varied to use alone, so the Census Bureau has its own set of geographic entities it uses to describe and sort the United States. See Census Basics: Geography: http://www.census.gov/mso/www/c2000basics/chapter4.htm for an explanation.
  
  
• Geographic boundaries change with every Census depending on population changes, needs of the Census Bureau and needs of the various local governments of the states. Census Tracts, for example, were first used in the Census Bureau's publications of tabulations in 1940. See Datasets: Decennial Census: http://govpubs.lib.umn.edu/census/affDScensus.phtml for listing of smallest geography for 2000 datasets.
• Zip Codes are not geographic areas. They are created to simplify things for the Post Office. Therefore, they may cover particular set of streets, or one building or even one floor in a building. The Census Bureau has tried to provide statistics by Zip Code in the past, but for 2000 they have moved to Zip Code Tabulation Areas, which are consistent geographic areas that closely correspond to actual zip codes.

Data Items

• Each Census is a snapshot of the U.S. on April 1 of the Census year. In 2000 all residents of the U.S. were asked about the following: age, race, sex, Hispanic or Latino origin, household relationship and whether the housing unit was owner- or renter-occupied.
• 1-in-6 residents received the Long Form, with these data items.
• The questions asked change with every Census. They may be dropped, modified or new ones added.
• The race category was the biggest change for 2000. For the first time ever, respondents were given the option of checking more than one race. As a result, 2000 data is not directly comparable to any previous data. (See Overview of Race and Hispanic Origin: http://www.census.gov/prod/2001pubs/c2kbr01-1.pdf)
  • Hispanic Origin is not a race. It's an ethnicity according to the Office of Budget and Management.
• Reporting of data items is limited to protect respondent privacy. The Census Bureau is required to keep any information that might identify a particular person secret for 72 years from the date of collection in the Decennial Census. Therefore, as the geographic unit decreases in size, privacy can only be maintained by limiting data on that area. (See Census 2000 Basics: Confidentiality: http://www.census.gov/mso/www/c2000basics/chapter3.htm)

Time

• Each Decennial Census has a unique set of questions and geographic boundaries. Therefore, using them as a source of time-series data is problematic. (See IPUMS USA: http://www.ipums.org for a discussion of what's involved in harmonizing data across Censuses)
• There is a significant delay between collection and release of data. It may take as many as four years for all data to be released:
  • Apportionment data: must be released within 9 months of collection (Dec. 31) in accordance with Title 13 U.S. Code
  • Redistricting data: must be released within 1 year in accordance with P.L. 94-171
  • All other data is released on a "flow" basis, beginning with Summary File 1 and ending with the Public Use Microdata Samples.