When patterns of nonresponse (either unit or item nonresponse) are significantly correlated with variables of interest in a survey, then the nonresponse contributes to biased estimates of those variables and is considered nonignorable. Recent trends of increasing survey nonresponse rates make the question whether nonresponse is ignorable or not more salient to more researchers.
Since data are only observed for responders, researchers often use participating sample members or members for whom there are complete responses to make inferences about a more general population. For example, a researcher estimating the average income of single parents might use income data observed for single-parent responders to make generalizations about average income for all single parents, including those who did not participate or who refused to answer the relevant questions. ...
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