Selection bias, also known as sampling bias, usually refers to groups (e.g., experimental, control) that are systematically different prior to experimental [Page 1490]manipulation or intervention due to the assignment of participants to groups. In other words, variations detected during a study are attributable to group differences due to selection bias or the independent variable (e.g., manipulated variable). Selection bias can occur during participant selection, assignment, and/or during the study. The bias that occurs during participant selection is generally identified as a threat to external validity, whereas bias that occurs during assignment is known as a threat to internal validity. During a study, if a significant number of participants withdraw without completing the study, selection bias can also occur. This entry examines the context in which ...
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