Selection Bias

Selection bias, also known as sampling bias, usually refers to groups (e.g., experimental, control) that are systematically different prior to experimental 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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