Berkson's paradox

Berkson's paradox is a statistical phenomenon that arises in epidemiological studies and other research settings. It refers to a situation where a statistical association between two variables is reversed or obscured when looking at a specific population or subgroup that is selected based on a third variable. The paradox was named after the statistician Joseph Berkson, who pointed out that in certain circumstances, conditioning on a variable can lead to misleading conclusions about the relationship between two other variables.