Simple examples to understand what confounders, colliders, mediators, and moderators are and how to “control for” variables in R with regression and propensity-score matching

In a hospital, 9% of all patients have Covid-19. But: Among the heavy smokers among these patients, only 6% have Covid-19. What? Does smoking reduce your risk of getting Covid? Another example: I recently saw a post on Twitter with a line graph showing that, in the UK, persons aged 18 to 59 who …