A measure derived by dividing the total number of males by the total number of females, and then multiplying by 100. This measure is rounded to the nearest tenth.

A question on the sex of individuals has been included in every census. Census 2000 was the first time that first name was used for imputation of cases where sex was not reported.

Public and private school includes people who attended school in the reference period and indicated they were enrolled by marking one of the questionnaire categories for either "public school, public college" or "private school, private college." Schools supported and controlled primarily by a federal, state, or local government are defined as public (including tribal schools). Those supported and controlled primarily by religious organizations or other private groups are private.

School enrollment questions have been included in the census since 1840; highest grade attended was first asked in 1940; type of school was first asked in 1960. Before 1940, the enrollment question in various censuses referred to attendance in the preceding 6 months or the preceding year. In 1940, the reference was to attendance in the month preceding the census, and in the 1950 and subsequent censuses, the question referred to attendance in the 2 months preceding the census date.

Until the 1910 census, there were no instructions limiting the kinds of schools in which enrollment was to be counted. Starting in 1910, the instructions indicated that attendance at "school, college, or any educational institution" was to be counted. In 1930 an instruction to include "night school" was added. In the 1940 instructions, night school, extension school, or vocational school were included only if the school was part of the regular school system. Correspondence school work of any kind was excluded. In the 1950 instructions, the term "regular school" was introduced, and it was defined as schooling which "advances a person towards an elementary or high school diploma or a college, university, or professional school degree." Vocational, trade, or business schools were excluded unless they were graded and considered part of a regular school system. On-the-job training was excluded, as was nursery school. Instruction by correspondence was excluded unless it was given by a regular school and counted towards promotion. In 1960, the question used the term "regular school or college" and a similar, though expanded, definition of "regular" was included in the instruction, which continued to exclude nursery school. Because of the use of mailed questionnaires in the 1960 census, it was the first census in which instructions were written for the respondent as well as enumerators. In the 1970 census, the questionnaire used the phrase "regular school or college" and included instructions to "count nursery school, kindergarten, and schooling that leads to an elementary school certificate, high school diploma, or college degree." Instructions in a separate document specified that to be counted as regular school, nursery school must include instruction as an important and integral phase of its program, and continued the exclusion of vocational, trade, and business schools. The 1980 census question was very similar to the 1970 question, but the separate instruction booklet did not require that nursery school include substantial instructional content in order to be counted. Instructions included in the 1990 respondent instruction guide, which was mailed with the census questionnaire, further specified that enrollment in a trade or business school, company training, or tutoring were not to be included unless the course would be accepted for credit at a regular elementary school, high school, or college. The instruction guide defines a public school as "any school or college controlled and supported by a local, county, state, or federal government." Schools supported and controlled primarily by religious organizations or other private groups were defined as private. In Census 2000 there was no separate instruction guide. The questionnaire reference book used by enumerators and telephone assistance staff contained these definitions for those who asked questions.

The age range for which enrollment data have been obtained and published has varied over the censuses. Information on enrollment was recorded for people of all ages in the 1930 and 1940 censuses and 1970 through 2000 censuses; for people under 30 years old in 1950; and for people 5 to 34 years old in 1960. Most of the published enrollment figures referred to people 5 to 20 years old in the 1930 census, 5 to 24 in 1940, 5 to 29 in 1950, 5 to 34 in 1970, and 3 years old and over in 1980 and later years. This growth in the age group whose enrollment was reported reflects increased interest in the number of children in preprimary schools and in the number of older people attending colleges and universities. In the 1950 and subsequent censuses, college students were enumerated where they lived while attending college; whereas, in earlier censuses, they generally were enumerated at their parental homes. This change should not affect the comparability of national figures on college enrollment since 1940; however, it may affect the comparability over time of enrollment figures at subnational levels.

Type of school was first introduced in the 1960 census, where a separate question asked the enrolled person whether he/she was in a "public" or "private" school. Beginning with the 1970 census, the type of school was incorporated into the response categories for the enrollment question and the terms were changed to "public," "parochial," and "other private." In the 1980 census, "private, church related" and "private, not church related" replaced "parochial" and "other private." In 1990 and 2000, "public" and "private" were used. Data on school enrollment also were collected and published by other federal, state, and local government agencies. Where these data were obtained from administrative records of school systems and institutions of higher learning, they were only roughly comparable to data from population censuses and household surveys because of differences in definitions and concepts, subject matter covered, time references, and enumeration methods. At the local level, the difference between the location of the institution and the residence of the student may affect the comparability of census and administrative data. Differences between the boundaries of school districts and census geographic units may also affect these comparisons.

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.")