Clustering illusion



Clustering illusion is the cognitive bias of seeing a pattern in what is actually a random sequence of numbers or events. It is a type of apophenia related to the gambler's fallacy.

A simple way to understand this illusion is to imagine casting ten pennies in a one foot square space. Unless all of the pennies fall in an exactly even distribution, which is extremely improbable, some pennies will be closer to each other than others and seem to form a cluster or group solely from the random distribution.

It's sometimes called the "hot hand fallacy" due to the belief common among basketball coaches and players that it was best to use players on a hot streak (i.e., those who had a "hot hand"). A study demonstrated that the hot hand was a matter of coaches picking a short run of baskets out of a larger sequence that was more or less random. Though there some dispute that the researchers defined "hot hand" (i.e., making exactly one basket following another) differently than basketball players actually conceptualize it (in a vaguer sense).

A similar idea cropped up among birders, known as the Patagonia Picnic Table Effect, that once a rare bird is spotted in one area, other rare birds will soon be spotted in the same area. An analysis of the birding database eBird, however, showed that this is unlikely to be a real effect.

Randomness is not such an intuitive concept for humans. In addition to being poor at recognizing random sequences for what they are, people are also bad at generating random numbers.