Confoundle · a reasoning trap
The Will Rogers phenomenon
Take the worst members of a good group and move them into a bad group. The good group's average rises, because its weakest ones left. The bad group's average rises too, because the newcomers are better than what it already had. Every group improves and nothing real has happened. It is how sharper scans can make survival look better in every stage of a disease while exactly as many people live and die.
The rule
Move members from one group into another and you can lift the average of every group at once, while the overall picture stays exactly the same.
What it looks like
Why it works
Picture two buckets, one of good outcomes and one of bad. Take the worst items out of the good bucket and drop them into the bad one, where they are the best of a bad lot. The good bucket's average rises because its weakest members left. The bad bucket's average rises because it gained members better than its own. Both averages improve and nothing about any individual has changed. In medicine the reshuffling is done by better scans, which find disease that was always there but previously invisible. That is why survival by stage can improve across the board in a period when the treatments themselves did not get better, and it is why comparing stages across eras of different technology is treacherous.
Source
See if it fools you
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