(e.g., going off or on a welfare program) between pairs of months that span two interviews (e.g., for SIPP, months 4-5, 8-9, 12-13, etc.) than between pairs of months for which data are collected from the same interview. The seam problem affects most variables for which monthly data are collected in SIPP—often strongly. For example, in the first year of the 1984 SIPP panel, over twice as many nonparticipants reported entering the Social Security program between seam months than nonseam months (Jabine, King, and Petroni, 1990: Table 6.2). The reasons for the occurrence and extent of the seam phenomenon are not well understood, but it clearly results in errors in the timing of transitions in SIPP and the duration of spells of program participation (and perhaps of poverty). It may or may not result in errors in the number of transitions that occur within a given period. For example, in the case of food stamps, total exits and entrances from SIPP are close to the rates derived from food stamp administrative records. In contrast, whether due to the seam effect or other factors, entrance rates from SIPP for SSI are significantly higher than those shown by program records (Jabine, King, and Petroni, 1990:59-60). The Census Bureau has pursued research and testing of alternative questionnaire designs and interviewing procedures that could reduce the seam problem and produce more accurate income reporting overall (see, e.g., Marquis, Moore, and Bogen, 1991). To date, there have been few positive results.

Aggregate Comparisons

Aggregate comparisons of income estimates from SIPP and CPS, like comparisons of internal indicators of data quality, show a mixed picture. On balance, SIPP seems to be doing a somewhat better job of income reporting, but not for all income types. Moreover, it may be that the gains in SIPP are not holding up over time.

Comparisons of 1984 estimates from the 1984 SIPP and March 1985 CPS showed SIPP as a percentage of CPS as follows (Jabine, King, and Petroni, 1990: Table 10.8):

Total money income

100.1

Regular money income

99.9

Earnings

98.2

All other

106.0

Public and private transfers

111.6

Property income

103.1

All other regular money income

37.0

Lump-sum payments

N.A. (not collected in CPS)

SIPP performed better than the March CPS with the notable exception of earnings. (The low ratio for all other regular money income is presumably due to higher levels of reporting of specific income types in SIPP than in the



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