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Simulated example showing the association between depressive symptoms and impulsivity (a) in the general population and (b) among people who use opioids. Opioid use is a collider, as it can be caused by depressive symptoms or impulsivity. Therefore, selecting a sample of opioid users produces a spurious negative association between depressive symptoms and impulsivity. [Colour figure can be viewed at wileyonlinelibrary.com]
