household nonresponse to the basic CPS, the Census Bureau increases the weights of responding households; to adjust for person nonresponse, it imputes a complete data record for another person with similar demographic characteristics. These procedures assume that respondents represent the characteristics of nonrespondents; this assumption has not been adequately tested.

Like all household surveys, SIPP experiences household nonresponse, and like all longitudinal surveys, it suffers cumulative sample loss or attrition at each successive interview wave (some households that fail to respond at an interview wave are subsequently brought back into the survey). In addition, it experiences "type Z" nonresponse—the failure to obtain information, either in person or by proxy, for individual members of otherwise cooperating households.

Attrition in SIPP to date has been highest at the first and second interviews—5-8 percent of eligible households at Wave 1 and 4-6 percent of eligible households at Wave 2. Thereafter, the additional loss is only 2-3 percent in each of Waves 3-5 and less than 1 percent in each subsequent wave. By Wave 6 (after 2 years of interviewing), cumulative sample loss is 18-20 percent of eligible households; by Wave 8, it is 21-22 percent (Bowie, 1991). The Panel to Evaluate SIPP estimated that total sample attrition at the end of 12 waves (4 years) might be 25 percent (Citro and Kalton, 1993:102). The attrition experience in SIPP is quite comparable to that in the ISDP (Nelson, Bowie, and Walker, 1987) and the PSID (with the exception that, as noted above, the PSID experienced a larger sample loss at the first two waves).

Attrition reduces the number of cases that are available for analysis—including the number available for longitudinal analysis over all or part of the time span of a panel and the number available for cross-sectional analysis from interview waves—and thereby increases the sampling error of the estimates. More important, the people who drop out may differ from those who remain in the survey. To the extent that adjustments to the weights for survey respondents do not compensate for these differences, estimates from the survey may be biased.

The available evidence does suggest that people who drop out of SIPP differ from those who stay in the survey. Studies of nonresponse from the 1984 SIPP panel show that household noninterview rates after the first wave tended to be higher for renters, for households located in large metropolitan areas, and for households headed by young adults. Individuals who did not complete all of the interview waves, compared with those who did, tended to include more residents of large metropolitan areas, renters, members of racial minorities, children and other relatives of the reference person, people aged 15-24, never-married people, and people with no savings accounts or other assets (Jabine, King, and Petroni, 1990:35-37, Table 5.4). A recent analysis of attrition from the 1990 SIPP panel obtained similar results (Lamas, Tin, and Eargle, 1994). This study found that attrition was more likely to occur among

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