When starting to work with the Census, you need to understand how the Census describes small-area geography. Large geography (Nation, State, County) is easy to understand, however, other geographic terms can be confusing for the novice because specialized terminology is used to describe small areas of the country. The Census Bureau standardizes this language across the US to facilitate comparison of geographic areas. The image below illustrates how the specialized terms fit together. Important tip: If doing historical census research, small area geographies began with larger urban areas in the 1940 Census and were not applied to the whole U.S. until the 2000 Census.
More on Census small area geography:
Blocks: The smallest geographic level for which census data is available. No population threshold and size varies.
Block Groups: Population of 300 to 4,000 (optimum size is 1,500) and follow physical boundaries.
Census Tracts: Population of 1,500 to 8,000 (optimum size is 4,000) and follow physical boundaries.
The Census Bureau collects a LOT of detailed personal information (income, family size, mortgage/rent paid, family relationships, etc). Because of this, the bureau maintains a level of confidentiality because the Census Bureau does not want to provide a way for any individual or household to be identified through the data it collects and publishes. While this may not matter with large geographies like Nation or State, it can become an issue with small areas like Blocks, Block Groups, and sometimes Tracts. When looking at small area geographies, you may run into instances where the statistic is footnoted with a phrase about confidentiality.
Census definitions of race have changed over time and reflect the anxieties and concerns of the time they were used. (Pew provides a great comparison of historical race/ethnicty questions with the 2010 Census). Currently only the following categories for race and ethnicity from the White House Office of Management and Budget (OMB) are used:
White – A person having origins in any of the original peoples of Europe, the Middle East, or North Africa.
Black or African American – A person having origins in any of the Black racial groups of Africa.
American Indian or Alaska Native – A person having origins in any of the original peoples of North and South America (including Central America) and who maintains tribal affiliation or community attachment.
Asian – A person having origins in any of the original peoples of the Far East, Southeast Asia, or the Indian subcontinent including, for example, Cambodia, China, India, Japan, Korea, Malaysia, Pakistan, the Philippine Islands, Thailand, and Vietnam.
Native Hawaiian or Other Pacific Islander – A person having origins in any of the original peoples of Hawaii, Guam, Samoa, or other Pacific Islands.
In addition to the five race categories, there are two for ethnicity: Hispanic or Latino and Not Hispanic or Latino. Hispanic/Latino is NOT a race -- Hispanics/Latinos can be of any race. The Census asks each person whether or not s/he is Hispanic/Latino, defined as a person of Cuban, Mexican, Puerto Rican, South or Central American, or other Spanish culture or origin regardless of race. (Note: People from Brazil are not included as Hispanic).
The following are some of the most searched census terms. If at any time you encounter a term that you do not understand or seems strange in the context, search it in the Census glossary.