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Selection Bias

Selection bias describes any threat to validity because subjects are included, treatment allocated, or outcomes measured for a nonrandom group or a nonrandom subset of the population to which a researcher wishes to generalize. This bias arises from the fact that the characteristics of participants that influence their inclusion in a study or selection into treatment may also affect the outcome being investigated. Selection bias is a central concern in empirical research in the economics of education, where researchers cannot perfectly control access to treatments of interest or measure all attributes of students or schools that affect the outcomes. This entry discusses the implications of selection bias for inference and describes three prominent types of selection bias. The final section discusses the role of research ...

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