Panel studies are subject to attrition, which is unit nonresponse after the initial wave of data collection. Attrition affects the results of analyses based on panel data by reducing the sample size and thereby diminishing the efficiency of the estimates. In addition, and more important, attrition also may be selective; differential or selective attrition occurs when the characteristics of the panel members who drop out of the panel because of attrition differ systematically from the characteristics of panel members who are retained in the panel. Differential attrition may introduce bias in survey estimates. However, the amount of bias depends both on the amount of attrition and on the selectivity of attrition, or in other words, on the association between the variables from which the estimate ...
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