This measure divides the age distribution into two equal parts: one-half of the cases falling below the median value and one-half above the value. Median age is computed on the basis of a single-year-of-age distribution using a linear interpolation method.

There is some tendency for respondents to provide their age as of the date they completed the census questionnaire or interview, not their age as of the census reference date. The two-part question and editing procedures have attempted to minimize the effect of this reporting problem on tabulations. Additionally, the current census age question displays the census reference date prominently, and interviewer training emphasizes the importance of collecting age as of the reference date.

Respondents sometimes round a persons age up if they were close to having a birthday. For most single years of age, the misstatements are largely offsetting. The problem is most pronounced at age 0. Also, there may have been more rounding up to age 1 to avoid reporting age as 0 years. (Age in completed months was not collected for infants under age 1.) Editing procedures correct this problem.

There is some respondent resistance to reporting the ages of babies in completed years (i.e., 0 years old when the baby is under 1 year old). Instead, babies ages are sometimes reported in months. The two-part question along with enhanced editing and data capture procedures correct much of this problem before the age data are finalized in tabulations. Additionally, the current census age question includes an instruction for babies ages to be answered as 0 years old when they are under 1 year old.

Age heaping is a common age misreporting error. Age heaping is the tendency for people to overreport ages (or years of birth) that end in certain digits (commonly digits 0 or 5) and underreport ages or years of birth ending in other digits. The two-part question helps minimize the effect of age heaping on the final tabulations.

Age data for centenarians have a history of data quality challenges. The counts in the 1970 and 1980 Censuses for people 100 years and over were substantially overstated. Editing and data collection methods have been enhanced in order to meet the data quality challenges for this population.

It also has been documented that the population aged 69 in the 1970 Census and the population aged 79 in the 1980 Census were overstated. The population aged 89 in 1990 and the population aged 99 in 2000 did not have an overstated count. (For more information on the design of the age question, see the Comparability section below.)

Age data have been collected in every census. However, there have been some differences in the way they have been collected and processed over time. In the 2010 Census (as in Census 2000), each individual provided both an age and an exact date of birth. The 1990 Census collected age and year of birth. Prior censuses had collected month and quarter of birth in addition to age and year of birth. The 1990 Census change was made so that coded information could be obtained for both age and year of birth.

In each census since 1940, the age of a person was assigned when it was not reported. In censuses before 1940, with the exception of 1880, people of unknown age were shown as a separate category. Since 1960, assignment of unknown age has been performed by a general procedure described as imputation. The specific procedures for imputing age have been different in each census. (For more information on imputation, see 2010 Census: Operational Overview and Accuracy of the Data.)

There are four changes to the Hispanic origin question for the 2010 Census. First, the wording of the question differs from that in 2000. In 2000, the question asked if the person was Spanish/Hispanic/Latino. In 2010, the question asks if the person is of Hispanic, Latino, or Spanish origin. Second, in 2000, the question provided an instruction, Mark ý the 'No'box if not Spanish/Hispanic/Latino. The 2010 Census question provided no specific instruction for non-Hispanics. Third, in 2010, the Yes, another Hispanic, Latino, or Spanish origin category provided examples of six Hispanic origin groups (Argentinean, Colombian, Dominican, Nicaraguan, Salvadoran, Spaniard, and so on) and instructed respondents to print origin. In 2000, no Hispanic origin examples were given. Finally, the fourth change was the addition of a new instruction in the 2010 Census that was not used in Census 2000. The instruction is stated as follows: NOTE: Please answer BOTH Question 8 about Hispanic origin and Question 9 about race. For this census, Hispanic origins are not races.

There were two changes to the Hispanic origin question for Census 2000. First, the sequence of the race and Hispanic origin questions for Census 2000 differs from that in 1990; in 1990, the race question preceded the Hispanic origin question. Testing prior to Census 2000 indicated that response to the Hispanic origin question could be improved by placing it before the race question without affecting the response to the race question. Second, there was an instruction preceding the Hispanic origin question indicating that respondents should answer both the Hispanic origin and the race questions. This instruction was added to give emphasis to the distinct concepts of the Hispanic origin and race questions, and emphasized the need for both pieces of information.

Furthermore, there was a change in the processing of the Hispanic origin and race responses. In the 1990 census, respondents provided Hispanic origin responses in the race question and race responses in the Hispanic origin question. In 1990, the Hispanic origin question and the race question had separate edits; therefore, although information may have been present on the questionnaire, it was not fully utilized due to the discrete nature of the edits. However, for Census 2000, there was a joint race and Hispanic origin edit that utilized Hispanic origin and race information regardless of the location.

A household includes all the people who occupy a housing unit. (People not living in households are classified as living in group quarters.) A housing unit is a house, an apartment, a mobile home, a group of rooms, or a single room that is occupied (or if vacant, is intended for occupancy) as separate living quarters. Separate living quarters are those in which the occupants live separately from any other people in the building and which have direct access from the outside of the building or through a common hall. The occupants may be a single family, one person living alone, two or more families living together, or any other group of related or unrelated people who share living arrangements. In the 2010 Census data products, the count of households or householders equals the count of occupied housing units.

A multigenerational household is one that contains three or more parent-child generations; for example, the householder, child of householder (either biological, stepchild, or adopted child), and grandchildren of householder. A householder with a parent or parent-in-law of the householder and a child of the householder may also be a multigenerational household.

An unmarried-partner household is a household other than a "husband-wife household" that includes a householder and an unmarried partner. An "unmarried partner" can be of the same sex or of the opposite sex as the householder. An "unmarried partner" in an "unmarried-partner household" is an adult who is unrelated to the householder but shares living quarters and has a close personal relationship with the householder. An "unmarried-partner" household also may be a family household or a nonfamily household, depending on the presence or absence of another person in the household who is related to the householder. There may be only one unmarried partner per household, and an unmarried partner may not be included in a husband-wife household, as the householder cannot have both a spouse and an unmarried partner. Same-sex married-couple households are edited into this category.

A person having origins in any of the original peoples of Europe, the Middle East, or North Africa. It includes people who indicate their race as White or report entries such as Irish, German, Italian, Lebanese, Arab, Moroccan, or Caucasian.

A person having origins in any of the Black racial groups of Africa. It includes people who indicate their race as Black, African Am., or Negro or report entries such as African American, Kenyan, Nigerian, or Haitian.

A person having origins in any of the original peoples of North and South America (including Central America) and who maintains tribal affiliation or community attachment. This category includes people who indicate their race as American Indian or Alaska Native or report entries such as Navajo, Blackfeet, Inupiat, Yupik, or Central American Indian groups or South American Indian groups.

Respondents who identified themselves as American Indian or Alaska Native were asked to report their enrolled or principal tribe. Therefore, tribal data in tabulations reflect the written entries reported on the questionnaires. Some of the entries (for example, Metlakatla Indian Community and Umatilla) represent reservations or a confederation of tribes on a reservation. The information on tribe is based on self-identification and, therefore, does not reflect any designation of federally or state-recognized tribe. The information for the 2010 Census was derived from the American Indian and Alaska Native Tribal Classification List for Census 2000 and updated from 2002 to 2009 based on the annual Federal Register notice entitled Indian Entities Recognized and Eligible to Receive Services From the United States Bureau of Indian Affairs, Department of the Interior, Bureau of Indian Affairs, issued by OMB, and through consultation with American Indian and Alaska Native communities and leaders.
The American Indian categories shown in Summary Files 1 and 2 represent tribal groupings, which refer to the combining of individual American Indian tribes, such as Fort Sill Apache, Mescalero Apache, and San Carlos Apache, into the general Apache tribal grouping.
The Alaska Native categories shown in Summary Files 1 and 2 represent tribal groupings, which refer to the combining of individual Alaska Native tribes, such as King Salmon Tribe, Native Village of Kanatak, and Sunaq Tribe of Kodiak, into the general Aleut tribal grouping.

A person having origins in any of the original peoples of the Far East, Southeast Asia, or the Indian subcontinent, including, for example, Cambodia, China, India, Japan, Korea, Malaysia, Pakistan, the Philippine Islands, Thailand, and Vietnam. It includes people who indicate their race as Asian Indian,Chinese,Filipino,Korean,Japanese,Vietnamese, and Other Asian or provide other detailed Asian responses.
    Asian Indian-Includes people who indicate their race as Asian Indian or report entries such as India or East Indian.
    Bangladeshi-Includes people who provide a response such as Bangladeshi or Bangladesh.
    Bhutanese-Includes people who provide a response such as Bhutanese or Bhutan.
    Burmese-Includes people who provide a response such as Burmese or Burma.
    Cambodian-Includes people who provide a response such as Cambodian or Cambodia.
    Chinese-Includes people who indicate their race as Chinese or report entries such as China or Chinese American. In some census tabulations, written entries of Taiwanese are included with Chinese, while in others they are shown separately.
    Filipino-Includes people who indicate their race as Filipino or report entries such as Philippines or Filipino American.
    Hmong-Includes people who provide a response such as Hmong or Mong.
    Indonesian-Includes people who provide a response such as Indonesian or Indonesia.
    Japanese-Includes people who indicate their race as Japanese or report entries such as Japan or Japanese American.
    Korean-Includes people who indicate their race as Korean or report entries such as Korea or Korean American.
Definitions of Subject Characteristics B-9U.S. Census Bureau, 2010 Census Summary File 1
    Laotian-Includes people who provide a response such as Laotian or Laos.
    Malaysian-Includes people who provide a response such as Malaysian or Malaysia.
    Nepalese-Includes people who provide a response such as Nepalese or Nepal.
    Pakistani-Includes people who provide a response such as Pakistani or Pakistan.
    Sri Lankan-Includes people who provide a response such as Sri Lankan or Sri Lanka.
    Taiwanese-Includes people who provide a response such as Taiwanese or Taiwan.
    Thai-Includes people who provide a response such as Thai or Thailand.
    Vietnamese-Includes people who indicate their race as Vietnamese or report entries such as Vietnam or Vietnamese American.
    Other Asian, specified-Includes people who provide a response of another Asian group, such as Iwo Jiman, Maldivian, Mongolian, Okinawan, or Singaporean.
    Other Asian, not specified-Includes respondents who checked the Other Asian response category on the census questionnaire and did not write in a specific group or wrote in a generic term such as Asian or Asiatic.

A person having origins in any of the original peoples of Hawaii, Guam, Samoa, or other Pacific Islands. It includes people who indicate their race as Native Hawaiian,Guamanian or Chamorro,Samoan, and Other Pacific Islander or provide other detailed Pacific Islander responses.
  Native Hawaiian-Includes people who indicate their race as Native Hawaiian or report entries such as Part Hawaiian or Hawaiian.
  Samoan-Includes people who indicate their race as Samoan or report entries such as American Samoan or Western Samoan.
  Tongan-Includes people who provide a response such as Tongan or Tonga.
  Other Polynesian-Includes people who provide a response of another Polynesian group, such as Tahitian, Tokelauan, or wrote in a generic term such as Polynesian.
  Guamanian or Chamorro-Includes people who indicate their race as Guamanian or Chamorro or report entries such as Chamorro or Guam.
  Marshallese-Includes people who provide a response such as Marshallese or Marshall Islands.
  Other Micronesian-Includes people who provide a response of another Micronesian group, such as Carolinian, Chuukese, I-Kiribati, Kosraean, Mariana Islander, Palauan, Pohnpeian, Saipanese, Yapese, or wrote in a generic term such as Micronesian.
  Fijian-Includes people who provide a response such as Fijian or Fiji.
  Other Melanesian-Includes people who provide a response of another Melanesian group, such as Guinean, Hebrides Islander, Solomon Islander, or wrote in a generic term such as Melanesian.
  Other Pacific Islander, not specified-Includes respondents who checked the Other Pacific Islander response category on the census questionnaire and did not write in a specific group or wrote in a generic term such as Pacific Islander.

Includes all other responses not included in the White,Black or African American,American Indian or Alaska Native,Asian, and Native Hawaiian or Other Pacific Islander race categories described above. Respondents reporting entries such as multiracial, mixed, interracial, or a Hispanic,
Latino, or Spanish group (for example, Mexican, Puerto Rican, Cuban, or Spanish) in response to the race question are included in this category.
B-10 Definitions of Subject CharacteristicsU.S. Census Bureau, 2010 Census Summary File 1

People may choose to provide two or more races either by checking two or more race response check boxes, by providing multiple responses, or by some combination of check boxes and other responses. The race response categories shown on the questionnaire are collapsed into the five minimum race groups identified by OMB and the Census Bureaus Some Other Race category. For data product purposes, Two or More Races refers to combinations of two or more of the following race categories:
  1. White
  2. Black or African American
  3. American Indian or Alaska Native
  4. Asian
  5. Native Hawaiian or Other Pacific Islander
  6. Some Other Race
There are 57 possible combinations (see Figure B-1) involving the race categories shown above. Thus, according to this approach, a response of "e;White"e; and "Asian" was tallied as Two or More Races, while a response of Japanese and Chinese was not because Japanese and Chinese are both Asian responses.

Figure B-1. Two or More Races (57 Possible Specified Combinations)
  1. White; Black or African American
  2. White; American Indian and Alaska Native
  3. White; Asian
  4. White; Native Hawaiian and Other Pacific Islander
  5. White; Some Other Race
  6. Black or African American; American Indian and Alaska Native
  7. Black or African American; Asian
  8. Black or African American; Native Hawaiian and Other Pacific Islander
  9. Black or African American; Some Other Race
  10. American Indian and Alaska Native; Asian
  11. American Indian and Alaska Native; Native Hawaiian and Other Pacific Islander
  12. American Indian and Alaska Native; Some Other Race
  13. Asian; Native Hawaiian and Other Pacific Islander
  14. Asian; Some Other Race
  15. Native Hawaiian and Other Pacific Islander; Some Other Race
  16. White; Black or African American; American Indian and Alaska Native
  17. White; Black or African American; Asian
  18. White; Black or African American; Native Hawaiian and Other Pacific Islander
  19. White; Black or African American; Some Other Race
  20. White; American Indian and Alaska Native; Asian
  21. White; American Indian and Alaska Native; Native Hawaiian and Other Pacific Islander
  22. White; American Indian and Alaska Native; Some Other Race
  23. White; Asian; Native Hawaiian and Other Pacific Islander
  24. White; Asian; Some Other Race
  25. White; Native Hawaiian and Other Pacific Islander; Some Other Race
  26. Black or African American; American Indian and Alaska Native; Asian
  27. Black or African American; American Indian and Alaska Native; Native Hawaiian and Other Pacific Islander
  28. Black or African American; American Indian and Alaska Native; Some Other Race
  29. Black or African American; Asian; Native Hawaiian and Other Pacific Islander
  30. Black or African American; Asian; Some Other Race
  31. Black or African American; Native Hawaiian and Other Pacific Islander; Some Other Race
  32. American Indian and Alaska Native; Asian; Native Hawaiian and Other Pacific Islander
  33. American Indian and Alaska Native; Asian; Some Other Race
  34. American Indian and Alaska Native; Native Hawaiian and Other Pacific Islander; Some Other Race
  35. Asian; Native Hawaiian and Other Pacific Islander; Some Other Race
  36. White; Black or African American; American Indian and Alaska Native; Asian
  37. White; Black or African American; American Indian and Alaska Native; Native Hawaiian and Other Pacific Islander
  38. White; Black or African American; American Indian and Alaska Native; Some Other Race
  39. White; Black or African American; Asian; Native Hawaiian and Other Pacific Islander
  40. White; Black or African American; Asian; Some Other Race
  41. White; Black or African American; Native Hawaiian and Other Pacific Islander; Some Other Race
  42. White; American Indian and Alaska Native; Asian; Native Hawaiian and Other Pacific Islander
  43. White; American Indian and Alaska Native; Asian; Some Other Race
  44. White; American Indian and Alaska Native; Native Hawaiian and Other Pacific Islander; Some Other Race
  45. White; Asian; Native Hawaiian and Other Pacific Islander; Some Other Race
  46. Black or African American; American Indian and Alaska Native; Asian; Native Hawaiian and Other Pacific Islander
  47. Black or African American; American Indian and Alaska Native; Asian; Some Other Race
  48. Black or African American; American Indian and Alaska Native; Native Hawaiian and Other Pacific Islander; Some Other Race
  49. Black or African American; Asian; Native Hawaiian and Other Pacific Islander; Some Other Race
  50. American Indian and Alaska Native; Asian; Native Hawaiian and Other Pacific Islander; Some Other Race
  51. White; Black or African American; American Indian and Alaska Native; Asian; Native Hawaiian and Other Pacific Islander
  52. White; Black or African American; American Indian and Alaska Native; Asian; Some Other Race
  53. White; Black or African American; American Indian and Alaska Native; Native Hawaiian and Other Pacific Islander; Some Other Race
  54. White; Black or African American; Asian; Native Hawaiian and Other Pacific Islander; Some Other Race
  55. White; American Indian and Alaska Native; Asian; Native Hawaiian and Other Pacific Islander; Some Other Race
  56. Black or African American; American Indian and Alaska Native; Asian; Native Hawaiian and Other Pacific Islander; Some Other Race
  57. White; Black or African American; American Indian and Alaska Native; Asian; Native Hawaiian and Other Pacific Islander; Some Other Race

Given the many possible ways of displaying data on race, data products will provide varying levels of detail. There are several additional concepts used to display race information for the six major race categories (White; Black or African American; American Indian or Alaska Native; Asian; Native Hawaiian or Other Pacific Islander; and Some Other Race) and the various details within these groups. The concept race "alone" includes people who reported a single entry (e.g., Korean) and no other race, as well as people who reported two or more entries within the same major race group (e.g., Asian). For example, respondents who reported Korean and Vietnamese are part of the larger "Asian alone" race group. The concept race alone or in combination includes people who reported a single race alone (e.g., Asian) and people who reported that race in combination with one or more of the other major race groups (e.g., White, Black or African American, American Indian and Alaska Native, Native Hawaiian and Other Pacific Islander, and Some Other Race). The concept race alone or in combination concept, therefore, represents the maximum number of people who reported as that race group, either alone or in combination with one or more additional race(s). The sum of the six individual race alone-or-in-combination categories may add to more than the total population because people who reported more than one race were tallied in each race category. The concept "race alone or in any combination" applies only to detailed race groups, such as American Indian and Alaska Native tribes, detailed Asian groups, and detailed Pacific Islander groups. For example, Korean alone or in any combination includes people who reported a single response (e.g., Korean), people who reported Korean and another Asian group (e.g., Korean and Vietnamese), and people who reported Korean in combination with one or more other non-Asian race groups (e.g., White, Black or African American, American Indian and Alaska Native, Native Hawaiian and Other Pacific Islander, or Some Other Race).

The 2010 Census included an automated review, computer edit, and coding operation on a 100 percent basis for the write-in responses to the race question, similar to that used in Census 2000. There were two types of coding operations: 1) automated coding where a write-in response was automatically coded if it matched a write-in response already contained in a database known as the master file and 2) expert coding, which took place when a write-in response did not match an entry already on the master file and was sent to expert coders familiar with the subject matter. During the coding process, subject-matter specialists reviewed and coded written entries from the response areas on the race question: American Indian or Alaska Native, Other Asian, Other Pacific Islander, and Some Other Race. Up to 30 text characters werecollected from each race write-in area, and up to two responses were coded and tabulated from each separate race write-in area.

There are three changes to the race question for the 2010 Census. First, the note to respondents was changed to read, Please answer BOTH Question 8 about Hispanic origin and Question 9 about race. For this census, Hispanic origins are not races. Second, the wording of the race question was changed from What is this persons race? Mark ý one or more races to indicate what this person considers himself/herself to be to What is Person 1s race? Mark ý one or more boxes. Third, examples were added to the Other Asian response categories (Hmong, Laotian, Thai, Pakistani, Cambodian, and so on) and the Other Pacific Islander response categories (Fijian, Tongan, and so on). The treatment of ethnic or national origin write-in responses to the race question also was different. For Census 2000, data on single ancestry by race from the 1990 census were used to help make decisions about how to code ethnic or national origin responses into one or more race categories. Essentially, if 90 percent or more of people who reported a single, specific ancestry reported in a specific race category in 1990 (e.g., more than 90 percent of people indicating Haitian ancestry reported as Black in the question on race), then that race was used as the Census 2000 response. This 90 percent rule was not applied to write-in responses of American Indian and Alaska Native tribes, Asian groups, or Pacific Islander groups because the question on race was designed explicitly to obtain these types of responses. For example, a write-in response of Haitian and Moroccan was coded as Black and Some Other Race.Moroccan was coded as Some Other Race because less than 90 percent of people indicating Moroccan ancestry reported in one specific race category in the question on race. For the 2010 Census, ethnic or national origin write-in responses to the race question were coded into one or more of the five OMB race categories, according to the 1997 OMB definitions of race. For example, a 2010 Census write-in response of Haitian and Moroccan was coded as Black and White following OMBs definitions. If it was not possible to determine which OMB race category the ethnic group or national origin should be coded into, it was included in the Some Other Race category. For more information about comparability to data collected in previous censuses, see Census 2000 Summary File 1 Technical Documentation prepared by the U.S. Census Bureau, 2001, at www.census.gov/cen2000/doc/sf1.pdf.

The sex ratio represents the balance between the male and female populations. Ratios above 100 indicate a larger male population, and ratios below 100 indicate a larger female population. This measure is derived by dividing the total number of males by the total number of females and then multiplying by 100. It is rounded to the nearest tenth.

A question on the sex of individuals has been asked of the total population in every census.