Money income. Data on income collected in the CPS are limited to money income received before payments for personal income taxes and deductions for Social Security, union dues, Medicare premiums, etc. Money income is the sum of the amounts received from earnings (including losses which occurred among the self-employed from their own farm or nonfarm operations); Social Security and public assistance payments; Supplemental Security income; dividends, interest, and rent (including losses); unemployment and worker's compensation; government and private employee pensions; and other periodic income. (Certain money receipts such as capital gains are not included.) Therefore, money income does not reflect the fact that many families receive part of their income in the form of nonmoney transfers such as food stamps, health benefits, and subsidized housing; that many farm families receive nonmoney income in the form of rent-free housing and goods produced and consumed on the farm; or that nonmoney incomes are also received by some nonfarm residents such as the use of business transportation and facilities, full or partial payments by business for retirement programs, medical and educational expenses, etc. These elements should be considered when comparing income levels.

As in most household surveys, the estimated total amount of income received by persons derived from the March CPS is somewhat less than estimated amounts derived from independent sources such as the Bureau of Economic Analysis, the Social Security Administration, and Veterans' Administration. The difference between the survey estimate and the independent estimate is termed "underreporting." Underreporting tends to be more pronounced for income sources that are not derived from earnings, such as Social Security. public assistance, and net income from interest, dividends. rentals, etc Overall income earned from wages or salary is

much better reported than other sources of income and, when dollar imputations are assigned for nonreporting, total wage and salary income approximates independently derived estimates. By contrast, 1978 income data (both reported and allocated) for Social Security and public assistance payments to beneficiaries were approximately 94 and 76 percent, respectively, of independently derived estimates. For further details, see Current Population Reports, Series P-60, No. 123. Poverty (low-income) classification. Families and unrelated individuals are classified as being above or below the poverty level using the poverty index adopted by a Federal Interagency Committee in 1969. This 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, sex and age of the family head, and farm-nonfarm residence. 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). The poverty threshold for a nonfarm famiv of four was $6.662 in 1978, about 7.6 percent higher than the comparable 1977 cutoff of $6,191. For further details, see Current Population Reports, Series P-60, No. 124. Symbols. A dash (-) represents zero or rounds to zero and the symbol "B" means that the base is less than 7500C. An "X" means not applicable.

SOURCE OF DATA

The estimates in this report are based on data collected in March and April of 1979 from the Current Population Survey (CPS) conducted by the Bureau of the Census. The present CPS sample was initially selected from the 1970 census file and is updated continuously to reflect new construction where possible. The sample was spread over 614 areas with coverage in each of the 50 States and the District of Columbia.

In the March sample, approximately 56,000 occupied households were eligible for interview. Interviews were not obtained at about 3,000 of these occupied units because the occupants were not found at home after repeated calls or were unavailable for some other reason. In addition to the 56,000 occupied units, about 10,000 sample addresses were visited but found to be vacant or otherwise ineligible for interview.

About 40,000 of the households interviewed in March were interviewed again in April. Women 18 years of age and older were asked supplemental questions regarding child support and alimony payments. These responses were successfully matched to the March responses for 37,033 of the 40,981 eligible women who were interviewed in March.' Child support and alimony information was imputed for the other 3,948 eligible women. The table below gives sample sizes and imputation rates by marital status.

Sample Size and Imputation Rates

RELIABILITY OF THE ESTIMATES

Since the estimates in this report are based on a sample, they may differ somewhat from the figures that would have been obtained if a complete census had been taken using the same schedules, instructions, and enumerators. There are two types of errors possible in an estimate based on a sample survey-sampling and nonsampling. The standard errors provided for this report primarily indicate the magnitude of the sampling error. They also partially measure the effect of some nonsampling errors in response and enumeration, but do not measure any systematic biases in the data. The full extent of the nonsampling error is unknown. Consequently, particular care should be exercised in the interpretation of figures based on a relatively small number of cases or on small differences between estimates.

Nonsampling variability. Nonsampling errors in surveys can be attributed to many sources, e.g., inability to obtain information about all cases in the sample, definitional difficulties, differences in the interpretation of questions, inability or unwillingness of respondents to provide correct information, inability to recall information, errors made in collection such as in recording or coding the data, errors made in processing the data, errors made in imputing values for missed data, and failure to represent all sample households and persons within sample households (undercoverage). Sampling variability. The standard errors given in this report are primarily measures of sampling variability, that is, of the variations that occurred by chance because a sample rather than the whole of the population was surveyed. The sample estimate and its estimated standard error enable one to construct interval estimates that include the average result of all possible samples with a known probability. For example, if all possible samples were selected, each of these surveyed under identical conditions and an estimate and its estimated standard error were calculated from each sample, then:

1. Approximately 68 percent of the intervals from one standard error below the estimate to one standard error above the estimate would include the average result of all possible samples;

2. 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;

3. Approximately 95 percent of the intervals from two standard errors below the estimate to two standard errors above the estimate would include the average result of all possible samples.

The average result of all possible samples may or may not be contained in any particular computed interval. However, for a particular sample one can say with specified confidence that the average result of all possible samples is included within the constructed interval.

All the statements of comparison appearing in the text are significant at a 1.6 standard error level or better, and most are significant at a level of more than 2.0 standard errors. This means that for most differences cited in the text, the estimated difference is greater than twice the standard error of the difference. Statements of comparison qualified in some way (e.g., by use of the phrase, "some evidence") have a level of a significance between 1.6 and 2.0 standard errors.

Note when using small estimates. Percent distributions are shown in the report only when the base of the percentage is 75,000 or greater. Because of the large standard errors involved, there is little chance that percentages would reveal useful information when computed on a smaller base. Estimated totals are shown, however, even though the relative standard errors of these totals are larger than those for corresponding percentages. These smaller estimates are provided primarily to permit such combinations of the categories as serve each user's needs.

Standard errors. Standard errors of estimates for those characteristics of greatest interest are given together with these estimates in tables 1 through 4. Standard errors for the other estimates can be computed directly by the user as outlined in the next section.

Computation of standard errors for estimated numbers and estimated percentages. Standard errors of estimated numbers

Here x is the size of the subclass of the population which is the base of the percentage, p is the percentage (0<p<100), and b is the parameter associated with the characteristic.

Table B-1 provides the values of the a and b parameters that are used in formulae (1) and (2) to create standard errors of estimated number of persons and estimated percentages.

Table B-1. Parameters To Be Used in the

Computation of Standard Errors for
Estimated Numbers and Percentages
of Persons