A new study in JAMA Ophthalmology considers whether eyeglasses confer protection from COVID-19. What it really demonstrates is how a variable -- in this case, eyeglasses -- can point in so many directions as to be useless. (But it will get you published in a peer-reviewed journal.)
Myopia or near-sightedness, and higher levels of education are associated. But which came first? And which is the cause? Can genetic information help us to convert correlation to causation while answering this chicken-or-egg question?