Data on school enrollment were derived from answers to long-form questionnaire Items 8a and 8b, which were asked of a sample of the population. People were classified as enrolled in school if they reported attending a "regular" public or private school or college at any time between February 1, 2000, and the time of enumeration. The question included instructions to "include only nursery school or preschool, kindergarten, elementary school, and schooling which leads to a high school diploma or a college degree" as regular school or college. Respondents who did not answer the enrollment question were assigned the enrollment status and type of school of a person with the same age, sex, and race/Hispanic or Latino origin whose residence was in the same or a nearby area.
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.