The data on marital status were derived from answers to long-form questionnaire Item 7, "What is this persons marital status," which was asked of a sample of the population. The marital status classification refers to the status at the time of enumeration. Data on marital status are tabulated only for the population 15 years old and over.

Each person was asked whether they were "Now married," "Widowed," "Divorced," "Separated," or "Never married." Couples who live together (for example, people in common-law marriages) were able to report the marital status they considered to be the most appropriate.

Never married includes all people who have never been married, including people whose only marriage(s) was annulled.

Ever married includes people married at the time of enumeration, along with those who are separated, widowed, or divorced.

Now married, except separated includes people whose current marriage has not ended through widowhood or divorce; or who are not currently separated. The category also may include people in common-law marriages if they consider this category the most appropriate. In certain tabulations, currently married people are further classified as "spouse present" or "spouse absent."

Separated includes people with legal separations, people living apart with intentions of obtaining a divorce, and people who are permanently or temporarily separated because of marital discord.

This category includes widows and widowers who have not remarried.

This category includes people who are legally divorced and who have not remarried.

All people whose current marriage has not ended by widowhood or divorce. This category includes people defined above as "separated."

Married people whose wives or husbands were enumerated as members of the same household or the same group quarters facility, including those whose spouses may have been temporarily absent for such reasons as travel or hospitalization.

Married people whose wives or husbands were not enumerated as members of the same household or the same group quarters facility.

Defined above.

Married people whose wives or husbands were not enumerated as members of the same household, excluding separated. For example, this includes any person whose spouse was employed and living away from home, in an institution, or away in the armed forces.

Differences between the number of currently married males and the number of currently married females occur because of reporting differences and because some husbands and wives have their usual residence in different areas. These differences also can occur because different weights are applied to the individuals data. Any differences between the number of "now married, spouse present" males and females are due solely to sample weighting procedures. By definition, the numbers would be the same.

Census 2000 marital status definitions are the same as those used in 1990. A general marital status question has been asked in every census since 1880. While the marital status question in Census 2000 is identical to that of 1990, in Census 2000 the question was only asked on the long form, while in previous years it was asked on the short form.

The data on age, which was asked of all people, were derived from answers to the long-form questionnaire Item 4 and short-form questionnaire Item 6. The age classification is based on the age of the person in complete years as of April 1, 2000. The age of the person usually was derived from their date of birth information. Their reported age was used only when date of birth information was unavailable.

Data on age are used to determine the applicability of some of the sample questions for a person and to classify other characteristics in census tabulations. Age data are needed to interpret most social and economic characteristics used to plan and examine many programs and policies. Therefore, age is tabulated by single years of age and by many different groupings, such as 5-year age groups.

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.")