March CPS.) Census Bureau analysts assume that many SIPP respondents are reporting their net paychecks rather than their gross earnings as requested by the survey.
Coder and Scoon-Rogers (1994) reported comparisons for detailed income sources for 1984 and 1990. These comparisons indicate that some of the gains in income reporting seen in SIPP at the outset of the survey may no longer be occurring. However, they noted that the 1990 SIPP panel may not be comparable to the 1984 panel because it contained an added sample, carried over from the 1989 panel, of households headed by single mothers and minorities. The weighting adjustments for these added cases may be problematic.
As with the review of internal indicators of data quality, it is difficult from the available comparisons of aggregates to draw conclusions about the implications for estimates of poverty and related income statistics. Perhaps the most telling summary indicator available is the fact, noted above, that SIPP poverty estimates are consistently several percentage points below those from the March CPS. Lamas, Tin, and Eargle (1994) found that only about one-sixth of this difference could be explained by attrition bias in SIPP. Another one-sixth of the difference appears due to more accurate measurement of family composition during the income reporting year in SIPP than in the March CPS. The remaining two-thirds difference, it is hypothesized, is explained by more complete reporting of income in SIPP for the lower end of the income distribution. In that regard, respondents to SIPP report more sources of income than respondents to the March CPS; they also report higher amounts for such income sources as Social Security, Railroad Retirement, SSI, unemployment compensation, veterans' payments, and child support payments, all of which are important to the low-income population. However, reporting of AFDC and other cash welfare is currently no more complete in SIPP than in the March CPS (Coder and Scoon-Rogers, 1994: Table 1). Clearly, much more analytical work needs to be done, including work to look at differences in income reporting among population groups within and across the surveys and the development of a complete time series of poverty and related income statistics from SIPP for comparison with the March CPS.