The data on ancestry were derived from answers to long-form questionnaire Item 10, which was asked of a sample of the population. The data represent self-classification by people according to the ancestry group or groups with which they most closely identify. Ancestry refers to a person's ethnic origin or descent, "roots," heritage, or the place of birth of the person, the persons parents, or their ancestors before their arrival in the United States. Some ethnic identities, such as Egyptian or Polish, can be traced to geographic areas outside the United States, while other ethnicities, such as Pennsylvania German or Cajun, evolved in the United States.

The intent of the ancestry question was not to measure the degree of attachment the respondent had to a particular ethnicity. For example, a response of "Irish" might reflect total involvement in an Irish community or only a memory of ancestors several generations removed from the individual. Also, the question was intended to provide data for groups that were not included in the Hispanic origin and race questions. Official Hispanic origin data come from long-form questionnaire Item 5, and official race data come from long-form questionnaire Item 6. Therefore, although data on all groups are collected, the ancestry data shown in these tabulations are for non-Hispanic and non-race groups. Hispanic and race groups are included in the "Other groups" category for the ancestry tables in these tabulations.

The ancestry question allowed respondents to report one or more ancestry groups, although only the first two were coded. If a response was in terms of a dual ancestry, for example, "Irish English," the person was assigned two codes, in this case one for Irish and another for English. However, in certain cases, multiple responses such as "French Canadian," "Greek Cypriote," and "Scotch Irish" were assigned a single code reflecting their status as unique groups. If a person reported one of these unique groups in addition to another group, for example, "Scotch Irish English," resulting in three terms, that person received one code for the unique group (Scotch-Irish) and another one for the remaining group (English). If a person reported "English Irish French," only English and Irish were coded. Certain combinations of ancestries where the ancestry group is a part of another, such as "German-Bavarian," were coded as a single ancestry using the more specific group (Bavarian). Also, responses such as "Polish-American" or "Italian-American" were coded and tabulated as a single entry (Polish or Italian).

The Census Bureau accepted "American" as a unique ethnicity if it was given alone, with an ambiguous response, or with state names. If the respondent listed any other ethnic identity such as "Italian-American," generally the "American" portion of the response was not coded. However, distinct groups such as "American Indian," "Mexican American," and "African American" were coded and identified separately because they represented groups who considered themselves different from those who reported as "Indian," "Mexican," or "African," respectively.

In all tabulations, when respondents provided an unclassifiable ethnic identity (for example, "multinational," "adopted," or "I have no idea"), the answer was included in tabulation category "Unclassified or not reported."

The tabulations on ancestry are presented using two types of data presentations - one using total people as the base, and the other using total responses as the base. The following are categories shown in the two data presentations.

Includes the second response of all people who reported a multiple ancestry. Thus, the count for Danish in this category includes all people who reported Danish as the second response, regardless of the first response provided.