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.

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.")

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.