Okay. Forget we mentioned that. Back to statistics. Selection effects like this one are sometimes known as observation selection effects because they are driven by an association between the very presence of the observer and the variable that the observer reports. In the class size example, if we ask students about the size of their classes, there is an association between the presence of the student observer and the class size. If instead we ask teachers about the sizes of their classes, there are no observation selection effects because each class has only one teacher—and therefore there is
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