Selection bias is often a challenging concept in epidemiology. One feature that distinguishes selection bias from confounding and measurement bias is, selection bias results from a change in the sample under study. It leads to a critical but, surprisingly, unsolved question. 1/

https://twitter.com/EpidemiologyLWW/status/1562411147546218496

Which sample/population does the selection bias refer to? The referent population before the selection process? Or the selected sample? 2/