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.

The 1990 census data products did not include a general disability status indicator. Furthermore, a comparable indicator could not be constructed since the conceptual framework of the 1990 census was more limited. The questionnaire included only three types of disability in questions with four subparts. The questions asked about whether an individual had a condition that had lasted for 6 months or more and that (1) limited the kind or amount of work that he or she could do at a job, (2) prevented the individual from working at a job, (3) made it difficult to go outside the home alone (for example, to shop or visit a doctors office), and (4) made it difficult to take care of his or her own personal needs, such as bathing, dressing, or getting around inside the home. The 1990 disability questions were asked on the long form questionnaire of the population 15 years old and over.

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.

This category includes people whose highest degree was a high school diploma or its equivalent, people who attended college but did not receive a degree, and people who received a college, university, or professional degree. People who reported completing the 12th grade but not receiving a diploma are not high school graduates.

This category includes people of compulsory school attendance age or above who were not enrolled in school and were not high school graduates. These people may be referred to as "high school dropouts." However, there is no criterion regarding when they "dropped out" of school, so they may have never attended high school.

From 1840 to 1930, the census measured educational attainment by means of a basic literacy question. In 1940, a single question was asked on highest grade of school completed. In the 1950 to 1980 censuses, a two-part question was used to construct highest grade or year of school completed. The question asked (1) the highest grade of school attended and (2) whether that grade was finished. For people who have not attended college, the response categories in the current educational attainment question should produce data that are comparable to data on highest grade completed from earlier censuses. For people who attended college, there is less comparability between years of school completed and highest degree.

Beginning in 1990, the response categories for people who have attended college were modified from earlier censuses because there was some ambiguity in interpreting responses in terms of the number of years of college completed. For instance, it was not clear whether "completed the fourth year of college," "completed the senior year of college," and "college graduate" were synonymous. Research conducted shortly before the 1990 census suggests that these terms were more distinct than in earlier decades, and this change may have threatened the ability to estimate the number of "college graduates" from the number of people reported as having completed the fourth or a higher year of college. It was even more difficult to make inferences about post-baccalaureate degrees and "Associate" degrees from highest year of college completed. Thus, comparisons of post-secondary educational attainment in the 2000 and 1990 censuses with data from the earlier censuses should be made with great caution.

Changes between 1990 and Census 2000 were slight. The two associate degree categories in 1990 were combined into one for Census 2000. "Some college, no degree" was split into two categories, "Some college credit, but less than 1 year," and "1 or more years of college, no degree." Prior to 1990, the college levels reported began with "Completed 1 year of college." Beginning in 1990, the first category was "Some college, no degree," which allowed people with less than 1 year of college to be given credit for college. Prior to 1990, they were included in "High school, 4 years." The two revised categories will accommodate comparisons with either data series and allow the tabulation of students who completed at least 1 year of college, as some data users wish. This will not change the total number who completed some college.

The category "12th grade, no diploma" was counted as high school completion or "Completed high school, 4 years" prior to 1990 and as "Less than high school graduate" in 1990 and 2000. In the 1960 and subsequent censuses, people for whom educational attainment was not reported were assigned the same attainment level as a similar person whose residence was in the same or a nearby area. In the 1940 and 1950 censuses, people for whom educational attainment was not reported were not allocated.

In censuses prior to 1990, "median school years completed" was used as a summary measure of educational attainment. Using the current educational attainment question, the median can only be calculated for groups of which less than half the members have attended college. "Percent high school graduate or higher" and "percent Bachelor's degree or higher" are summary measures that can be calculated from the present data and offer quite readily interpretable measures of differences between population subgroups.