Regression to the mean

Regression to the mean is a technical way of saying that things tend to even out over time.

The sprinter that breaks the world record will probably run closer to their average time on the next race, or the medical treatment that achieves stunning results on the first trial will probably not be as efficacious on the second.

Specifically, it refers to the tendency of a random variable that is highly distinct from the norm to return to "normal" over repeated tests.

On average, observations tend to cluster around the mean (forming a normal distribution)¹

It only becomes most obvious when a strange result (e.g. a hole-in-one in golf) is followed by something much more ordinary (like a double-bogey).

Regression to the mean forms the basis for the Central Limit Theorem (CLT)

Footnotes

1. whether or not they follow an unusual value. ↩