Nonresponse bias occurs when sampled elements from which data are gathered are different on the measured variables in nonnegligible ways from those that are sampled but from which data are not gathered. Essentially, all surveys are likely to have some degree of nonresponse bias, but in many cases it occurs at a very small and thus a negligible (i.e. ignorable) level. The size of the bias is a function of (a) the magnitude of the difference between respondents and nonrespondents and (b) the proportion of all sampled elements that are nonrespondents. Thus, even if only one of these factors is large, the nonresponse bias may well be nonnegligible.
There are three basic types of survey nonresponse. The first is refusals, which occur when sampled individuals or ...
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