[dm319]

It's a known statistical phenonema (Will Rogers phenomenon) [1] used to illustrate the problems when adjustments are made to clusters.

[1] https://en.wikipedia.org/wiki/Will_Rogers_phenomenon

[nojs]

Concrete example from that article: with improved detection of cancer, it’s true that “people with cancer live longer on average” and also that “people without cancer live longer on average” even if there’s no change to the treatment or anyone’s lifespan.