The tabulation of school enrollment by employment status is similar to that published in 1980 and 1990 census reports. The 1980 census tabulation included a single data line for armed forces; school enrollment, educational attainment, and employment status data were shown for the civilian population only. In 1970, a tabulation was included for 16 to 21 year old males not attending school.

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.