Heavy-tailed distribution

In probability theory, heavy-tailed distributions are probability distributions whose tails are not exponentially bounded: that is, they have heavier tails than the exponential distribution. In many applications it is the right tail of the distribution that is of interest, but a distribution may have a heavy left tail, or both tails may be heavy.

There are two important subclasses of heavy-tailed distributions, the long-tailed distributions and the subexponential distributions. In practice, all commonly used heavy-tailed distributions belong to the subexponential class.

There is still some discrepancy over the use of the term heavy-tailed. There are two other definitions in use. Some authors use the term to refer to those distributions which do not have all their power moments finite; and some others to those distributions that do not have a variance. The definition given in this article is the most general in use, and includes all distributions encompassed by the alternative definitions, as well as those distributions such as log-normal that possess all their power moments, yet which are generally acknowledged to be heavy-tailed. (Occasionally, heavy-tailed is used for any distribution that has heavier tails than the normal distribution.)

Definition of heavy-tailed distribution

The distribution of a random variable X with distribution function F is said to have a heavy right tail if

$\lim_{x \to \infty} e^{\lambda x}\Pr[X>x] = \infty \quad \mbox{for all } \lambda>0.\,$

This is also written in terms of the tail distribution function

$\overline{F}(x) \equiv \Pr(X>x) \,$

as

$\lim_{x \to \infty} e^{\lambda x}\overline{F}(x) = \infty \quad \mbox{for all } \lambda>0.\,$

This is equivalent to the statement that the moment generating function of F, M_F(t), is infinite for all t > 0.

The definitions of heavy-tailed for left-tailed or two tailed distributions are similar.

Definition of long-tailed distribution

The distribution of a random variable X with distribution function F is said to have a long right tail if for all t > 0,

$\lim_{x \to \infty} \Pr[X>x+t|X>x] =1, \,$

or equivalently

$\overline{F}(x+t) \sim \overline{F}(x) \quad \mbox{as } x \to \infty. \,$

This has the intuitive interpretation for a right-tailed long-tailed distributed quantity that if the long-tailed quantity exceeds some high level, the probability approaches 1 that it will exceed any other higher level: if you know the situation is bad, it is probably worse than you think.

All long-tailed distributions are heavy-tailed, but the converse is false, and it is possible to construct heavy-tailed distributions that are not long-tailed.

Subexponential distributions

Subexponentiality is defined in terms of convolutions of probability distributions. For two independent, identically distributed random variables X₁,X₂ with common distribution function F the convolution of F with itself, F ^{* 2} is defined, using Lebesgue-Stieltjes integration, by:

$\Pr(X_1+X_2 \leq x) = F^{*2}(x) = \int_{- \infty}^{\infty} F(x-y)\,dF(y).$

The n-fold convolution F ^{* n} is defined in the same way. The tail distribution function $\overline{F}$ is defined as $\overline{F}(x) = 1-F(x)$.

A distribution F on the positive half-line is subexponential if

$\overline{F^{*2}}(x) \sim 2\overline{F}(x) \quad \mbox{as } x \to \infty.$

This implies that, for any $n \geq 1$,

$\overline{F^{*n}}(x) \sim n\overline{F}(x) \quad \mbox{as } x \to \infty.$

The probabilistic interpretation of this is that, for a sum of n independent random variables $X_1,\ldots,X_n$ with common distribution F,

$\Pr(X_1+ \cdots X_n>x) \sim \Pr(\max(X_1, \ldots,X_n)>x) \quad \mbox{as } x \to \infty.$

This is often known as the principle of the single big jump.

A distribution F on the whole real line is subexponential if the distribution $F I([0,\infty))$ is. Here $I([0,\infty))$ is the indicator function of the positive half-line. Alternatively, a random variable X supported on the real line is subexponential if and only if X ⁺ = max(0,X) is subexponential.

All subexponential distributions are long-tailed, but examples can be constructed of long-tailed distributions that are not subexponential.

Common heavy-tailed distributions

All commonly used heavy-tailed distributions are subexponential.

Those that are one-tailed include:

• the Pareto distribution;
• the Log-normal distribution;
• the Lévy distribution;
• the Weibull distribution with shape parameter less than 1;
• the Burr distribution;
• the Log-gamma distribution;
• the Log-Cauchy distribution, sometimes described as having a "super-heavy tail" because it exhibits logarithmic decay producing a heavier tail than the Pareto distribution.

Those that are two-tailed include:

• The Cauchy distribution, itself a special case of both the stable distribution and the t-distribution;
• The family of stable distributions, excepting the special case of the normal distribution within that family. Some stable distributions are one-sided (or supported by a half-line), see e.g. Lévy distribution. See also financial models with long-tailed distributions and volatility clustering.
• The t-distribution.
• The skew lognormal cascade distribution.

See also

• Fat tail
• Leptokurtic
• Outlier
• The Long Tail
• Power Law