Primary sources are first-hand or contemporary accounts of an event or topic. They enable the researcher to get as close as possible to what actually happened during a historical event or time period.
Secondary sources add interpretation, analysis, or commentary to an event or phenomenon covered by primary sources.
A comprehensive summary of statistics on the social, political, and economic organization of the United States, consisting of over 1400 indexed tables, published since 1878. Serves as a statistical reference and guide to additional sources of information.