Data on ability to speak English were derived from the answers to long-form questionnaire Item 11c, which was asked of a sample of the population. Respondents who reported that they spoke a language other than English in long-form questionnaire Item 11a were asked to indicate their ability to speak English in one of the following categories: "Very well," "Well," "Not well," or "Not at all."
The data on ability to speak English represent the persons own perception about his or her own ability or, because census questionnaires are usually completed by one household member, the responses may represent the perception of another household member. Respondents were not instructed on how to interpret the response categories in Question 11c.
People who reported that they spoke a language other than English at home, but whose ability to speak English was not reported, were assigned the English-language ability of a randomly selected person of the same age, Hispanic origin, nativity and year of entry, and language group.
A household in which no person 14 years old and over speaks only English and no person 14 years old and over who speaks a language other than English speaks English "Very well" is classified as "linguistically isolated." In other words, a household in which all members 14 years old and over speak a non-English language and also speak English less than "Very well" (have difficulty with English) is "linguistically isolated." All the members of a linguistically isolated household are tabulated as linguistically isolated, including members under 14 years old who may speak only English.
The current question on ability to speak English was asked for the first time in 1980. From 1890 to 1910, "Able to speak English, yes/no" was asked along with two literacy questions. In tabulations from 1980, the categories "Very well" and "Well" were combined. Data from other surveys suggested a major difference between the category "Very well" and the remaining categories. In some tabulations showing ability to speak English, people who reported that they spoke English "Very well" are presented separately from people who reported their ability to speak English as less than "Very well."