March Current Population Survey Income Supplement
The Current Population Survey (CPS) is a voluntary survey of monthly labor force participation, begun in the 1940s, that includes supplemental questions in many months, including the annual March income supplement, which is used to estimate WIC eligibility. The March CPS asks household respondents about income received during the previous calendar year from the following sources: earnings, unemployment compensation, workers’ compensation, Social Security, Supplemental Security Income, public assistance, veterans’ payments, survivor benefits, disability benefits, pension or retirement income, interest, dividends, rents, royalties, and estates and trusts, educational assistance, alimony, child support, financial assistance from outside the household, and other income. While income data are reported for the previous calendar year, it is important to note that demographic and household membership data are reported as of the date of the survey. Thus, income information reported in the survey might not reflect the income of all the members of the household at the time of the survey because the composition of the household might change between the period for which income is measured and the period for which demographic and household membership data are measured. The March CPS Supplement does not ask questions about WIC participation; however, a separate CPS supplement, the Food Security Supplement, which has been conducted every year since 1995 (in a month other than March), includes questions about WIC participation of sampled households. The March
CPS Supplement is the input dataset for the Transfer Income Microsimulation (TRIM) model discussed in Appendix D.
The monthly CPS sample includes about 50,000 households, or 1 in 2,000 of all U.S. households. The monthly CPS has a rotating panel design, under which each sampled address is in the survey for 4 months, out of the survey for 8 months, and in the survey for another 4 months. Threefourths of the sample addresses are common from one month to the next, and one-half are common for the same month a year earlier. The sample for the March CPS Supplement consists of the basic monthly CPS sample and an additional sample of Hispanic households.
The CPS uses a multistage probability sample design, which is revised after each decennial census. The CPS has a state representative design, which results in larger states generally having larger CPS sample sizes, but with the largest states having CPS sample sizes that are smaller than their proportionate share of the U.S. population and the smallest states having proportionately larger sample sizes. In fall 1999 the Census Bureau received an appropriation to adjust the March CPS sample size and design so that reliable annual estimates at the state level could be provided of the numbers of low-income children lacking health insurance coverage by family income, age, and race or ethnicity.
Data collection for the CPS is carried out by permanent, experienced interviewers. The first and fifth interviews at an address are usually conducted in person; the other six interviews at an address are usually conducted by telephone. One household member who is age 15 or older is allowed to respond for other members.
Like other household surveys, the CPS exhibits population undercoverage at higher rates than the census. For March 1994, the ratio of the CPS estimated population to the census-based population control total (all ages) was 92 percent; for black men age 30–44 years, the coverage ratios were as low as 67–68 percent in 1994 (U.S. Census Bureau, 1996:Table D-2). It is estimated that about two-thirds of CPS undercoverage is due to missed people in otherwise interviewed households (i.e., people whose existence, let alone any information about them, is not known to the interviewer); the remainder is due to missed housing units because the address was not included in the sampling frame. CPS undercoverage is corrected by ratio adjustments to the survey weights that bring the CPS estimates of population in line with updated national population controls by age, race, sex, and Hispanic origin. Beginning with the March 1994 CPS, the population controls for survey weights reflect an adjustment for the undercount in the
census itself. However, the ratio adjustments do not correct for other characteristics, such as income, on which the undercovered population might be expected to differ from the covered population in each adjustment cell.
There is substantial item nonresponse in the March income supplement. Household income is one such item for which nonresponse is substantial. Imputation techniques are used to provide values for people who fail to respond to the income supplement entirely, as well as for people who fail to answer one or more questions on the supplement.
Additional information can be found at the Census Bureau web site for the CPS (http://www.bls.census.gov/cps/ads/adsmain.htm).