berkson's paradox
1. example
- To be accepted you must have an SAT score above a threshhold. Among accepted university students, there is a negative correlation between math and verbal scores, because you are only considering people in the "upper triangle" of the graph with a high enough SAT score, which can be achieved by getting high math + verbal, high math, or high verbal.
- https://en.wikipedia.org/wiki/Berkson%27s_paradox