The federal census has been taken every 10 years (decennially) since 1790. The taking of the census is established in Article 1, Section 2 of the United States Constitution. The most recent decennial census for which data is available was taken in 2010.
The Census Bureau reports data for a wide variety of geographic types, ranging from the entire United States down to a Census Block. Some geographic types follow recognized boundaries, such as counties and cities. Others are created by the Census Bureau.
Geography: Reference (Census Bureau)
The diagram shows the many geographic types for which data are available. In general, larger geographic types (e.g., state) are shown near the top and smaller geographic types (e.g., census tract) are shown towards the bottom.
Census Geography: "Help" in Explore Census Data.
Statistical Abstract has long been the authoritative resource for statistics. This is the single most important source for starting a statistical search. Statistical Abstract collects data for all facets of social, economic, and political life and includes all federal sources, some commercial publishers, and some professional and trade organizations. Tables have notes about sources of the data which can lead to other statistics. Tables are available in .pdf and Excel (.xls) formats.
Beginning with the 2013 edition, it is available in database format.
Statistical Abstract of the United States (Online Edition) (ProQuest)
This book examines census information, such as instructions to enumerators. These instructions led to undercounting women and misrepresenting women's occupational status.