tail event

tail event

English

Each of the two cylinders contain a tail, as understood in the context of sense 1: only the sequence members that come after some index.

The orange parts represent the sense of tail used in sense 2: the part of a distribution farthest from the main body. The exact point at which the "tail" begins is entirely arbitrary.

Etymology

tail +‎ event

Noun

tail event (plural tail events)

1. (probability) an event associated with a given infinite sequence ${\\displaystyle Y=\\{y_{k}|k\\geq k_{0}\\}}$ that is determined by any subsequence of the form ${\\displaystyle Y_{n}=\\{y_{k}|k\\geq n\\},n\\geq k_{0}}$ (a "tail" of that sequence)
2. a low-probability event
   • 2012, Masahiro Kawai, Eswar S. Prasad, New Paradigms for Financial Regulation: Emerging Market Perspectives, Brookings Institution Press (→ISBN), page 186:

     Exactly two years later, it was shattered by the force of a tsunami equivalent to the impact of 250 jumbo jets flying at 1,000 kilometers an hour—the very apotheosis of a real tail event.

   • 2013, Mark Spitznagel, The Dao of Capital: Austrian Investing in a Distorted World, John Wiley & Sons (→ISBN), page 243:

     ... When the MS index is high ex ante, subsequent large stock market losses and crashes are no longer tail events at all—rather they are perfectly expected events.

   • 2012, Masahiro Kawai, Eswar S. Prasad, New Paradigms for Financial Regulation: Emerging Market Perspectives, Brookings Institution Press (→ISBN), page 185:

     Policy frameworks anticipating only “ordinary” shocks within the usual economic cycles may not be very effective in mitigating the impact of such tail events when they actually materialize.

3. an event that initiates an activity

See also

• (tail of sequence): Kolmogorov's zero-one law