• Unit nonresponse. Unit nonresponse occurs when a sampled unit (person or family) fails to participate in the survey. Unit nonresponse can occur, for example, because the sampled person cannot be located, refuses to participate, is too ill to participate, cannot participate because of language or hearing problems, or is away from the area for the period of the survey fieldwork.

  • Item nonresponse. Item nonresponse occurs when a sampled unit participates in the survey but fails to provide responses to one or more of the survey questions. This failure may occur because the respondent refuses to answer a question on the grounds that it is too sensitive or personal, or because he or she does not know the answer to the question. Item nonresponse also occurs when an interviewer fails to ask a question or record the answer and when recorded responses are deleted in editing a questionnaire because the response is inconsistent with the answers recorded for other questions.

There is a potential for bias whenever sampled persons who did not participate in the survey have different characteristics than those who did. For some important characteristics, the respondents may be substantially different from those with missing data. In fact, if such differences exist and no attempt is made to adjust for them in the analyses, estimates or inferences for the target population may be misleading. The potential for bias is particularly great when nonresponse rates are high. Thus, for example, if those recently enrolled are not included on the sampling frame for enrollees in a state’s welfare program, the survey clearly will produce biased estimates of the distribution of length of time on the program, and any other associated estimates. Similarly, in a survey of welfare leavers, it may be that those who participate in the survey have had appreciably different experiences than those who do not, and thus, estimates based on the respondents will be biased. Suppose that families with positive outcomes (those who successfully made the transition from welfare) are easier to locate and more willing to respond than families with negative outcomes. In fact, policy makers are concerned that this situation does exist and that nonresponding and nonlocatable families and those whose current status is no longer reflected in administrative data are worse off and at greater risk than families for whom data are available (U.S. General Accounting Office, 1999). This situation can result in estimates with large nonresponse bias.

The standard method of attempting to reduce the potentially biasing effect of noncoverage and of unit nonresponse is a “weighting adjustment.” Weighting adjustments for these two sources of missing data are described in this paper. Because some state surveys have experienced high nonresponse rates, nonresponse weighting adjustments are likely to be particularly important. The intent of this paper is to describe how they may be applied.

All methods for handling missing data aim to reduce their potential biasing effects, but these methods cannot be expected to eliminate the effects of missing data. The best protection against potential nonresponse bias is to plan and imple-



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