Researchers who use survey data often assume that nonresponse (either unit or item nonresponse) in the survey is ignorable. That is, data that are gathered from responders to the survey are often used to make inferences about a more general population. This implies that the units with missing or incomplete data are a random subsample of the original sample and do not differ from the population at large in any appreciable (i.e. meaningful and nonignorable) way. By definition, if nonresponse is ignorable for certain variables, then it does not contribute to bias in the estimates of those variables.
Because nonresponse error (bias) is a function of both the nonresponse rate and the difference between respondents and nonrespondents on the statistic of interest, it is possible for ...
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