Berkson's Paradox

A counterintuitive result in conditional probability and statistics where there is a false observation of a negative correlation between two positive traits, i.e., that members of a population which have some positive trait tend to lack a second. For example, a person may observe from their experience that fast food restaurants in their area which serve good hamburgers tend to serve bad fries and vice versa; but because they would likely not eat anywhere where both were bad, they fail to allow for the large number of restaurants in this category which would weaken or even flip the correlation.