A housing unit is owner occupied if the owner or co-owner lives in the unit even if it is mortgaged or not fully paid for. The owner or co-owner must live in the unit and usually is Person 1 on the questionnaire. The unit is "Owned by you or someone in this household with a mortgage or loan" if it is being purchased with a mortgage or some other debt arrangement, such as a deed of trust, trust deed, contract to purchase, land contract, or purchase agreement. The unit is also considered owned with a mortgage if it is built on leased land and there is a mortgage on the unit. Mobile homes occupied by owners with installment loans balances are also included in this category.

A housing unit is "Owned by you or someone in this household free and clear (without a mortgage or loan)" if there is no mortgage or other similar debt on the house, apartment, or mobile home including units built on leased land if the unit is owned outright without a mortgage.

The tenure item on the Census 2000 questionnaire distinguishes between units owned with a mortgage or loan and those owned free and clear. In the sample data products, as in the 100-percent products, the tenure item provides data for total owner-occupied units. Detailed information that identifies mortgaged and nonmortgaged units are provided in other sample housing matrices. (For more information, see discussion under "Mortgage Status," "Selected Monthly Owner Costs," and "Selected Monthly Owner Costs as a Percentage of Household Income in 1999.")

All occupied housing units that are not owner occupied, whether they are rented for cash rent or occupied without payment of cash rent, are classified as renter occupied. "No cash rent" units are separately identified in the rent tabulations. Such units are generally provided free by friends or relatives or in exchange for services, such as resident manager, caretaker, minister, or tenant farmer. Housing units on military bases also are classified in the "No cash rent" category. "Rented for cash rent" includes units in continuing care, sometimes called life care arrangements. These arrangements usually involve a contract between one or more individuals and a service provider guaranteeing the individual shelter, usually a house or apartment, and services, such as meals or transportation to shopping or recreation. (For more information, see "Meals Included in Rent.")

Data on tenure have been collected since 1890. For 1990, the response categories were expanded to allow the respondent to report whether the unit was owned with a mortgage or loan, or free and clear (without a mortgage). The distinction between units owned with a mortgage and units owned free and clear was added in 1990 to improve the count of owner-occupied units. Research after the 1980 census indicated some respondents did not consider their units owned if they had a mortgage. In Census 2000, we continued with the same tenure categories used in the 1990 census.

Median age divides the age distribution into two equal parts: one-half of the cases falling below the median age and one-half above the median. Median age is computed on the basis of a single year of age standard distribution (see the "Standard Distributions" section under "Derived Measures"). Median age is rounded to the nearest tenth. (For more information on medians, see "Derived Measures".)

The most general limitation for many decades has been the tendency of people to overreport ages or years of birth that end in zero or 5. This phenomenon is called "age heaping." In addition, the counts in the 1970 and 1980 censuses for people 100 years old and over were substantially overstated. So also were the counts of people 69 years old in 1970 and 79 years old in 1980. Improvements have been made since then in the questionnaire design and in the imputation procedures that have minimized these problems.

Review of detailed 1990 census information indicated that respondents tended to provide their age as of the date of completion of the questionnaire, not their age as of April 1, 1990. One reason this happened was that respondents were not specifically instructed to provide their age as of April 1, 1990. Another reason was that data collection efforts continued well past the census date. In addition, there may have been a tendency for respondents to round their age up if they were close to having a birthday. It is likely that approximately 10 percent of people in most age groups were actually 1 year younger. For most single years of age, the misstatements were largely offsetting. The problem is most pronounced at age zero because people lost to age 1 probably were not fully offset by the inclusion of babies born after April 1, 1990. Also, there may have been more rounding up to age 1 to avoid reporting age as zero years. (Age in complete months was not collected for infants under age 1.)

The reporting of age 1 year older than true age on April 1, 1990, is likely to have been greater in areas where the census data were collected later in calendar year 1990. The magnitude of this problem was much less in the 1960, 1970, and 1980 censuses where age was typically derived from respondent data on year of birth and quarter of birth.

These shortcomings were minimized in Census 2000 because age was usually calculated from exact date of birth and because respondents were specifically asked to provide their age as of April 1, 2000. (For more information on the design of the age question, see the section below that discusses "Comparability.")

Age data have been collected in every census. For the first time since 1950, the 1990 data were not available by quarter year of age. This change was made so that coded information could be obtained for both age and year of birth. In 2000, each individual has both an age and an exact date 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 "Accuracy of the Data.")

Median year structure built divides the distribution into two equal parts: one-half of the cases falling below the median year structure built and one-half above the median. Median year structure built is computed on the basis of a standard distribution (see the "Standard Distributions" section under "Derived Measures"). Median year structure built is rounded to the nearest whole number. Median age of housing can be obtained by subtracting median year structure built from 2000. For example, if the median year structure built is 1967, the median age of housing in that area is 33 years (2000 minus 1967). (For more information on medians, see "Derived Measures".)

Data on year structure built are more susceptible to errors of response and nonreporting than data on many other items because respondents must rely on their memory or on estimates by people who have lived in the neighborhood a long time.

Data on year structure built were collected for the first time in the 1940 census. Since then, the response categories have been modified to accommodate the 10-year period between each census. In the 1980 census, the number of units built before 1940 appeared to be underreported. In an effort to alleviate this problem, a "Don't know" category was added in 1990. Responses of "Don't know" were treated like blanks and the item was allocated from similar units by tenure and structure type. However, this led to an extremely high allocation rate for the item (28 percent). A 1996 test proved inconclusive in determining whether a "Don't know" category led to a more accurate count of older units, but the test showed the allocation rate for this item was greatly reduced by the elimination of the "Don't know" category. As a result, "Don't know" was deleted for Census 2000.