The questions on educational attainment apply only to progress in "regular" schools. Such schools include graded public, parochial or other private elementary and high schools (both junior and senior high), colleges, universities, and professional schools, whether day schools or night schools. Thus, regular schooling is that which may advance a person toward an elementary school certificate or high school diploma, or a college, university, or professional school degree. Schooling in other than regular schools was counted only if the credits obtained were regarded as transferable to a school in the regular school system.

Labor force. Persons are classified as in the labor force if they were employed as civilians, unemployed, or in the Armed Forces during the survey week. The "civilian labor force" is comprised of all civilians 15 years old and over classified as employed or unemployed.

Paid labor force. Persons are classified as in the paid labor force if they were employed as wage and salary workers or self-employed workers during the survey week or were looking for work at the time and had last worked as wage and salary or self-employed workers. Employed. Employed persons comprise (1) all civilians who, during the survey week, did any work at all as paid employees or in their own business or profession, or on their own farm, or who worked 15 hours or more as unpaid workers on a farm or in a business operated by a member of the family, and (2) all those who were not working but who had jobs or businesses from which they were temporarily absent because of illness, bad weather, vacation, or labor-management dispute, or because they were taking time off for personal reasons, whether or not they were paid by their employers for time off, and whether or not they were seeking other jobs. Excluded from the employed group are persons whose only activity consisted of work around the house (such as own home housework, painting or repairing own home) or volunteer work for religious, charitable, and similar organizations.

Unemployed. Unemployed persons are those civilians who, during the survey week, had no employment but were available for work and (1) had engaged in any specific job seeking activity within the past 4 weeks, such as registering at a public or private employment office, meeting with prospective employers, checking with friends or relatives, placing or answering advertisements, writing letters of application, or being on a union or professional register; (2) were waiting to be called back to a job from which they had been laid off; or (3) were waiting to report to a new wage or salary job within 30 days.

Occupation. The data on occupation of employed persons 16 years old and over refer to the civilian job held

during the survey week. Persons employed at two or more jobs were reported in the job at which they worked the greatest number of hours during the week.

In 1980, the Bureau of the Census revised the Standard Occupational Classification System (SOC) for use in its tabulation program for the 1980 census and subsequent published reports on occupational data. Consequently, the new classification system was incorporated into the CPS tabulation program in January 1983. While the new system provides comparability between the CPS and other data sources, it causes a break in continuity for all CPS series containing occupational data.

Differences between the 1970 and 1980 occupational systems affect classifications at all levels. Such commonly used identifiers as white-collar, blue-collar, professional and technical, craft workers, and operative occupations have been eliminated. These identifiers have been replaced with new categories which represent conceptual as well as language changes. Moreover, many of the components of the former groupings have been shifted to such an extent that they cannot be made to correspond readily to the new categories. For a more complete explanation and description of the changes from the old to the new occupational classification system see the February 1983 issue of "Employment and Earnings" by the Bureau of Labor Statistics.

The occupation classification system developed for the 1980 census consists of 503 specific occupation categories arranged into 6 summary and 13 major occupation groups. The major occupation groups are combined in this report into 6 summary groups as follows:

Managerial and professional specialty occupations

Executive, administrative, and managerial occupations
Professional specialty occupations

Technical, sales, and administrative support occupations
Technicians, and related support occupations
Sales occupations

Administrative support occupations, including clerical

Service occupations

Private household occupations
Protective service occupations

Service occupations, except protective and household

Farming, forestry, and fishing occupations
Precision production, craft, and repair occupations

Operators, fabricators, and laborers

Machine operators, assemblers, and inspectors Transportation and material moving occupations Handlers, equipment cleaners, helpers, and laborers

Income. For each person 15 years old and over in the sample, questions were asked on the amount of money income received in the preceding calendar year from each of the following sources: (1) money wages or salary; (2) net income from nonfarm self-employment; (3) net income from farm self-employment; (4) Social

Security or railroad retirement; (5) Supplemental Security income; (6) public assistance or welfare payments; (7) interest (on savings or other investments which pay interest); (8) dividends, income from estates or trusts, or net rental income; (9) veterans' payments or unemployment and worker's compensation; (10) private pensions or government employee pensions; (11) alimony or child support, regular contributions from persons not living in the household, and other periodic income.

Although the income statistics refer to receipts during the preceding year the characteristics of the person, such as age, labor force status, etc., and the composition of families refer to the time of the survey. The income of the family does not include amounts received by persons who were members of the family during all or part of the income year if these persons no longer resided with the family at the time of enumeration. However, family income includes amounts reported by related persons who did not reside with the family during the income year but who were members of the family at the time of enumeration.

Data on consumer income collected in the CPS by the Bureau of the Census cover money income received (exclusive of certain money receipts such as capital gains) before payments for personal income taxes, Social Security, union dues, Medicare deductions, etc. Therefore, money income data do not reflect the fact that some families receive part of their income in the form of noncash benefits such as food stamps, health benefits, and subsidized housing; that some farm families receive noncash benefits in the form of rent-free housing and goods produced and consumed on the farm; or that noncash benefits are also received by some nonfarm residents which often take the form of the use of business transportation and facilities, full or partial payments by business for retirement programs, and medical and educational expenses, etc. These elements should be considered when comparing income levels. (For a detailed explanation of noncash benefits, see Current Population Reports, Series P-60, No. 155, Receipt of Selected Noncash Benefits: 1985.) Moreover, for many different reasons, there is a tendency in household surveys for respondents to underreport their income. From an analysis of independently derived income estimates, it has been determined that income earned from wages or salaries is much better reported than other sources of income, and is nearly equal to independent estimates of aggregate income. For a detailed explanation, see Current Population Reports, Series P-60 No. 162, Money Income of Households, Families, and Persons in the United States: 1987.

Money earnings. Money earnings are the algebraic sum of money wages or salary and net income from farm and nonfarm self-employment. For a detailed

explanation, see Current Population Reports, Series P-60, No. 162, Money Income of Households, Families, and Persons in the United States: 1987.

Number of earners. This number includes all persons in the family with $1 or more in wages and salaries, or $1 or more or a loss in net income from farm or nonfarm self-employment.

Poverty definition. Families and unrelated individuals are classified as being above or below the poverty level using the poverty index originated at the Social Security Administration in 1964 and revised by Federal Interagency Committees in 1969 and 1980. The poverty index is based solely on money income and does not reflect the fact that many low-income persons receive noncash benefits such as food stamps, Medicaid, and public housing. The index is based on the Department of Agriculture's 1961 Economy Food Plan and reflects the different consumption requirements of families based on their size and composition. It was determined from the Department of Agriculture's 1955 Survey of Food Consumption that families of three or more persons spend approximately one-third of their income on food; the poverty level for these families was, therefore, set at three times the cost of the Economy Food Plan. For smaller families and persons living alone, the cost of the Economy Food Plan was multiplied by factors that were slightly higher in order to compensate for the relatively larger fixed expenses of these smaller households. The poverty thresholds are updated every year to reflect changes in the Consumer Price Index (CPI-U). For example, the average poverty threshold for a family of four was $12,675 in 1989, $12,091 in 1988, and $11,611 in 1987. For a detailed explanation of the poverty definition, see Current Population Reports, Series P-60, No. 168, Money Income and Poverty Status in the United States: 1989.

Median. The median is presented in connection with the data on age, years of school completed, and income. It is the value which divides the distribution into two equal parts, one-half of the cases falling below this value and one-half of the cases exceeding this value.

Mean. The mean (average) is presented in connection with data on number of persons per family, income of persons, and income of families. The mean number of persons per family is the value obtained by dividing the number of persons in families having the characteristic under consideration by the appropriate number of families. The mean income is the amount obtained by dividing the total income of a group by the number of units in that group. The mean for families are based on all families. The mean for persons are based on persons with income.

SOURCE OF DATA

Most estimates in this report come from data obtained in March of 1990 in the Current Population Survey (CPS). The Bureau of the Census conducts the survey every month, although this report uses only March data for its estimates. Also, some estimates come from 1980 decennial census data. The March survey uses two sets of questions, the basic CPS and the supplement.

Basic CPS. The basic CPS collects primarily labor force data about the civilian noninstitutional population. Interviewers ask questions concerning labor force participation about each member 15 years old and over in every sample household.

The March 1990 CPS sample was selected from the 1980 decennial census files with coverage in all 50 States and the District of Columbia. The sample is continually updated to account for new residential construction. It is located in 729 areas comprising 1,973 counties, independent cities, and minor civil divisions. About 60,000 occupied households are eligible for interview every month. Interviewers are unable to obtain interviews at about 2,600 of these units because the occupants are not home after repeated calls or are unavailable for some other reason.

Since the introduction of the CPS, the Bureau of the Census has redesigned the CPS sample several times to improve the quality and reliability of the data and to satisfy changing data needs. The most recent changes were completely implemented in July 1985.

The following table summarizes changes in the CPS designs for the years for which data appear in this report.

March Supplement. In addition to the basic CPS questions, interviewers asked supplementary questions in March about the economic situation of persons and families for the previous year.

To obtain more reliable data for the Hispanic-origin population, the March CPS sample was increased by about 2,500 eligible housing units, interviewed the previous November, that contained at least one sample person of Hispanic origin. In addition, the sample included persons in the Armed Forces living off post or with their families on post.

Estimation Procedure. This survey's estimation procedure inflates weighted sample results to independent estimates of the civilian noninstitutional population of the United States by age, sex, race and Hispanic/nonHispanic categories. The independent estimates were based on statistics from decennial censuses of population; statistics on births, deaths, immigration and emigration; and statistics on the size of the Armed Forces. The independent population estimates used for 1981 (1980 for income estimates) to present were based on updates to controls established by the 1980 decennial census. Data previous to 1981 were based on independent population estimates from the most recent decennial census. For more details on the change in independent estimates, see the section entitled "Introduction of 1980 Census Population Controls" in an earlier report (Series P-60, No. 133). The estimation procedure for the March supplement included a further adjustment so husband and wife of a household received the same weight.

The estimates in this report for 1982 and later also employ a revised survey weighting procedure for persons of Hispanic origin. In previous years, weighted sample results were inflated to independent estimates of the noninstitutional population by age, sex, and race. There was no specific control of the survey estimates for the Hispanic population. Since then, the Bureau of the Census developed independent population controls for the Hispanic population by sex and detailed age

groups. Revised weighting procedures incorporate these new controls. The independent population estimates include some, but not all, undocumented immigrants.

ACCURACY OF ESTIMATES

Since the CPS estimates come from a sample, they may differ from figures from a complete census using the same questionnaires, instructions, and enumerators. A sample survey estimate has two possible types of error: sampling and nonsampling. The accuracy of an estimate depends on both types of error, but the full extent of the nonsampling error is unknown. Consequently, one should be particularly careful when interpreting results based on a relatively small number of cases or on small differences between estimates. The standard errors for CPS estimates primarily indicate the magnitude of sampling error. They also partially measure the effect of some nonsampling errors in responses and enumeration, but do not measure systematic biases in the data. (Bias is the average over all possible samples of the differences between the sample estimates and the desired value.)

Nonsampling Variability. Nonsampling errors can be attributed to many sources. These sources include the inability to obtain information about all cases in the sample, definitional difficulties, differences in the interpretation of questions, respondents' inability or unwillingness to provide correct information or to recall information, errors made in data collection such as in recording or coding the data, errors made in processing the data, errors made in estimating values for missing data, and failure to represent all units with the sample (undercoverage).

CPS undercoverage results from missed housing units and missed persons within sample households. Compared to the level of the 1980 Decennial Census, overall CPS undercoverage is about 7 percent. CPS undercoverage varies with age, sex, and race. Generally, undercoverage is larger for males than for females and larger for Blacks and other races combined than for Whites. As described previously, ratio estimation to independent age-sexrace-Hispanic population controls partially corrects for the bias due to undercoverage. However, biases exist in the estimates to the extent that missed persons in missed households or missed persons in interviewed households have different characteristics from those of interviewed persons in the same age-sex-raceHispanic group. Furthermore, the independent population controls have not been adjusted for undercoverage in the 1980 census.

For additional information on nonsampling error including the possible impact on CPS data when known, refer to Statistical Policy Working Paper 3, An Error Profile: Employment as Measured by the Current Population

Survey, Office of Federal Statistical Policy and Standards, U.S. Department of Commerce, 1978; and Technical Paper 40, The Current Population Survey: Design and Methodology, Bureau of the Census, U.S. Department of Commerce.

Sampling Variability. Sampling variability is variation that occurred by chance because a sample was surveyed rather than the entire population. Standard errors, as calculated by methods described later in "Standard Errors and Their Use," are primarily measures of sampling variability, although they may include some nonsampling error.

Comparability of Data. Data obtained from the CPS and other sources are not entirely comparable. This results from differences in interviewer training and experience and in differing survey processes. This is an example of nonsampling variability not reflected in the standard errors. Use caution when comparing results from different sources.

Caution should also be used when comparing estimates in this report, which reflect 1980 census-based population controls, with estimates for 1980 (1979 for income estimates) and earlier years, which reflect 1970 census-based population controls. This change in population controls had relatively little impact on summary measures such as means, medians, and percentage distributions, but did have a significant impact on levels. For example, use of 1980 based population controls results in about a 2-percent increase in the civilian noninstitutional population and in the number of families and households. Thus, estimates of levels for data collected in 1981 and later years will differ from those for earlier years by more than what could be attributed to actual changes in the population. These differences could be disproportionately greater for certain subpopulation groups than for the total population.

Since no independent population control totals for persons of Hispanic origin were used before 1982, compare Hispanic estimates over time cautiously.

Note When Using Small Estimates. Summary measures (such as medians and percentage distributions) are shown only when the base is 75,000 or greater. Because of the large standard errors involved, summary measures would probably not reveal useful information when computed on a smaller base. However, estimated numbers are shown even though the relative standard errors of these numbers are larger than those for corresponding percentages. These smaller estimates permit combinations of the categories to suit data users' needs. Take care in the interpretation of small differences. For instance, even a small amount of nonsampling error can cause a borderline difference to appear significant or not, thus distorting a seemingly valid hypothesis test.

Standard Errors and Their Use. A number of approximations are required to derive, at a moderate cost, standard errors applicable to all the estimates in this report. Instead of providing an individual standard error for each estimate, generalized sets of standard errors are provided for various types of characteristics. Thus, the tables show levels of magnitude of standard errors rather than the precise standard errors.

The sample estimate and its standard error enable one to construct a confidence interval, a range that would include the average result of all possible samples with a known probability. For example, if all possible samples were surveyed under essentially the same general conditions and using the same sample design, and if an estimate and its standard error were calculated from each sample, then approximately 90 percent of the intervals from 1.6 standard errors below the estimate to 1.6 standard errors above the estimate would include the average result of all possible samples.

A particular confidence interval may or may not contain the average estimate derived from all possible samples. However, one can say with specified confidence that the interval includes the average estimate calculated from all possible samples.

Some statements in the report may contain estimates followed by a number in parentheses. This number can be added to and subtracted from the estimate to calculate upper and lower bounds of the 90 percent confidence interval. For example, if a statement contains the phrase "grew by 1.7 percent (±1.0)," the 90 percent confidence interval for the estimate, 1.7 percent, is 0.7 percent to 2.7 percent.

Standard errors may also be used to perform hypothesis testing, a procedure for distinguishing between population parameters using sample estimates. The most common type of hypothesis appearing in this report is that the population parameters are different. An example of this would be comparing the average size of Hispanic families in 1990 to the average size of Hispanic families in 1989.

Tests may be performed at various levels of significance, where a significance level is the probability of concluding that the characteristics are different when, in fact, they are the same. All statements of comparison in the text have passed a hypothesis test at the 0.10 level of significance or better. This means that the absolute value of the estimated difference between characteristics is greater than or equal to 1.6 times the standard error of the difference.

Standard Errors of Estimated Numbers. There are two ways to compute the approximate standard error, Sx, of an estimated number shown in this report. The first uses the formula

Sx = fs