Missing data for a particular question or item is called item nonresponse. It occurs when a respondent fails to provide an answer to a required item. The ACS also considers invalid answers as item nonresponse. The Census Bureau uses imputation methods that either use rules to determine acceptable answers or use answers from similar housing units or people who provided the item information. One type of imputation, allocation, involves using statistical procedures, such as within-household or nearest neighbor matrices populated by donors, to impute for missing values.

Data on language spoken at home were derived from answers to the 2008 American Community Survey Questions 13a and 13b. These questions were asked only of persons 5 years of age and older. Instructions mailed with the American Community Survey questionnaire instructed respondents to mark "Yes" on Question 13a if they sometimes or always spoke a language other than English at home, and "No" if a language was spoken only at school - or if speaking was limited to a few expressions or slang. For Question 13b, respondents printed the name of the non-English language they spoke at home. If the person spoke more than one non-English language, they reported the language spoken most often. If the language spoken most frequently could not be determined, the respondent reported the language learned first.

Questions 13a and 13b referred to languages spoken at home in an effort to measure the current use of languages other than English. This category excluded respondents who spoke a language other than English exclusively outside of the home.

Most respondents who reported speaking a language other than English also spoke English. The questions did not permit a determination of the primary language of persons who spoke both English and another language.

An automated computer system coded write-in responses to Question 13b into more than 380 detailed language categories. This automated procedure compared write-in responses with a master computer code list - which contained approximately 55,000 previously coded language names and variants - and then assigned a detailed language category to each write-in response. The computerized matching assured that identical alphabetic entries received the same code. Clerical coding categorized any write-in responses that did not match the computer dictionary. When multiple languages other than English were specified, only the first was coded.
The write-in responses represented the names people used for languages they spoke. They may not have matched the names or categories used by professional linguists. The categories used were sometimes geographic and sometimes linguistic. The following table provides an illustration of the content of the classification schemes used to present language data.
Four and Thirty-Nine Group Classifications of Languages Spoken at Home with Illustrative Examples

Four-Group Classification	Thirty-nine Group Classification	Examples
Spanish	Spanish or Spanish Creole	Spanish, Ladino, Pachuco
Other Indo-European languages	French	French, Cajun, Patois
	French Creole	Haitian Creole
	Italian	Italian
	Portuguese or Portuguese Creole	Portuguese, Papia Mentae
	German	German, Luxembourgian
	Yiddish	Yiddish
	Other West Germanic languages	Dutch, Pennsylvania Dutch, Afrikaans
	Scandinavian languages	Danish, Norwegian, Swedish
	Greek	Greek
	Russian	Russian
	Polish	Polish
	Serbo-Croatian	Serbo-Croatian, Croatian, Serbian
	Other Slavic languages	Czech, Slovak, Ukrainian
	Armenian	Armenian
	Persian	Persian
	Gujarathi	Gujarathi
	Hindi	Hindi
	Urdu	Urdu
	Other Indic languages	Bengali, Marathi, Punjabi, Romany
	Other Indo-European languages	Albanian, Gaelic, Lithuanian,Rumanian
Asian and Pacific Island languages	Chinese	Cantonese, Formosan, Mandarin
	Japanese	Japanese
	Korean	Korean
	Mon-Khmer, Cambodian	Mon-Khmer, Cambodian
	Hmong	Hmong
	Thai	Thai
	Laotian	Laotian
	Vietnamese	Vietnamese
	Other Asian languages	Dravidian languages (Malayalam, Telugu, Tamil),Turkish
	Tagalog	Tagalog
	Other Pacific Island languages	Chamorro, Hawaiian, Ilocano, Indonesian, Samoan
All other languages	Navajo	Navajo
	Other Native North American languages	Apache, Cherokee, Dakota, Pima, Yupik
	Hungarian	Hungarian
	Arabic	Arabic
	Hebrew	Hebrew
	African languages	Amharic, Ibo, Twi, Yoruba, Bantu, Swahili, Somali
	Other and unspecified languages	Syriac, Finnish, Other languages of the Americas, not reported

The data on age were derived from answers to Question 4. The age classification is based on the age of the person in complete years at the time of interview. Both age and date of birth are used in combination to calculate the most accurate age at the time of the interview. Inconsistently reported and missing values are assigned or imputed based on the values of other variables for that person, from other people in the household, or from people in other households ("hot deck" imputation). Data on age are used to determine the applicability of other questions for a particular individual and to classify other characteristics in tabulations. Age data are needed to interpret most social and economic characteristics used to plan and analyze programs and policies. Therefore, age data are tabulated by many different age groupings, such as 5-year age groups.