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In [[finance]], '''volatility clustering''' refers to the observation, first noted
[https://doi.org/10.1016/0927-5398(93)90006-D A long memory property of stock market returns and a new model], Journal of Empirical Finance,
Volume 1, Issue 1, 1993, Pages 83-106</ref> and Barndorff-Nielsen and Shephard.<ref>{{cite encyclopedia |author= Ole E. Barndorff-Nielsen, Neil Shephard|
</ref>
Observations of this type in financial time series go against simple random walk models and have led to the use of [[GARCH]] models and mean-reverting [[stochastic volatility]] models in financial forecasting and [[Derivative (finance)|derivatives]] pricing. The [[ARCH]] ([[Robert F. Engle|Engle]], 1982) and
▲In [[finance]], '''volatility clustering''' refers to the observation, first noted as [[Benoît Mandelbrot|Mandelbrot]] (1963), that "large changes tend to be followed by large changes, of either sign, and small changes tend to be followed by small changes."<ref>Mandelbrot, B. B., [[https://www.jstor.org/stable/2351623| The Variation of Certain Speculative Prices]], The Journal of Business 36, No. 4, (1963), 394-419</ref> A quantitative manifestation of this fact is that, while returns themselves are uncorrelated, absolute returns <math>|r_{t}|</math> or their squares display a positive, significant and slowly decaying autocorrelation function: corr(|r{{sub|t}}|, |r{{sub|t+τ}} |) > 0 for τ ranging from a few minutes to several weeks.<ref>Cont, Rama (2005). "[https://doi.org/10.1007/1-84628-048-6_11 Long range dependence in financial markets]". In Lévy-Véhel J., Lutton E. (eds) Fractals in Engineering. Springer, London. pp. 159–179.</ref>
▲Observations of this type in financial time series have led to the use of [[GARCH]] models and mean-reverting [[stochastic volatility]] models in financial forecasting and [[Derivative (finance)|derivatives]] pricing. The [[ARCH]] ([[Robert F. Engle|Engle]], 1982) and [[GARCH]] ([[Tim Bollerslev|Bollerslev]], 1986) models aim to more accurately describe the phenomenon of volatility clustering and related effects such as [[kurtosis]]. The main idea behind these two models is that volatility is dependent upon past realizations of the asset process and related volatility process. This is a more precise formulation of the intuition that asset [[Volatility (finance)|volatility]] tends to revert to some mean rather than remaining constant or moving in [[monotonic]] fashion over time.
==See also==
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==References==
{{Reflist}}
▲{{cite encyclopedia |author= Ole E. Barndorff-Nielsen, Neil Shephard|title= Volatility|encyclopedia= Encyclopedia of Quantitative Finance|date= October 2010 |year= 2010 |publisher= Wiley|editor-last= Cont|editor-first=Rama |doi=10.1002/9780470061602.eqf19019 }}
{{Volatility}}
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