# Lomax distribution

The Lomax distribution, conditionally also called the Pareto Type II distribution, is a heavy-tail probability distribution used in business, economics, actuarial science, queueing theory and Internet traffic modeling.[1][2][3] It is named after K. S. Lomax. It is essentially a Pareto distribution that has been shifted so that its support begins at zero.[4]

Parameters Probability density function Cumulative distribution function ${\displaystyle \alpha >0}$ shape (real) ${\displaystyle \lambda >0}$ scale (real) ${\displaystyle x\geq 0}$ ${\displaystyle {\alpha \over \lambda }\left[{1+{x \over \lambda }}\right]^{-(\alpha +1)}}$ ${\displaystyle 1-\left[{1+{x \over \lambda }}\right]^{-\alpha }}$ ${\displaystyle {\lambda \over {\alpha -1}}{\text{ for }}\alpha >1}$ Otherwise undefined ${\displaystyle \lambda ({\sqrt[{\alpha }]{2}}-1)}$ 0 ${\displaystyle {{\lambda ^{2}\alpha } \over {(\alpha -1)^{2}(\alpha -2)}}{\text{ for }}\alpha >2}$ ${\displaystyle \infty {\text{ for }}1<\alpha \leq 2}$ Otherwise undefined ${\displaystyle {\frac {2(1+\alpha )}{\alpha -3}}\,{\sqrt {\frac {\alpha -2}{\alpha }}}{\text{ for }}\alpha >3\,}$ ${\displaystyle {\frac {6(\alpha ^{3}+\alpha ^{2}-6\alpha -2)}{\alpha (\alpha -3)(\alpha -4)}}{\text{ for }}\alpha >4\,}$

## Characterization

### Probability density function

The probability density function (pdf) for the Lomax distribution is given by

${\displaystyle p(x)={\alpha \over \lambda }\left[{1+{x \over \lambda }}\right]^{-(\alpha +1)},\qquad x\geq 0,}$

with shape parameter ${\displaystyle \alpha >0}$ and scale parameter ${\displaystyle \lambda >0}$ . The density can be rewritten in such a way that more clearly shows the relation to the Pareto Type I distribution. That is:

${\displaystyle p(x)={{\alpha \lambda ^{\alpha }} \over {(x+\lambda )^{\alpha +1}}}}$ .

### Non-central moments

The ${\displaystyle \nu }$ th non-central moment ${\displaystyle E[X^{\nu }]}$ exists only if the shape parameter ${\displaystyle \alpha }$ strictly exceeds ${\displaystyle \nu }$ , when the moment has the value

${\displaystyle E(X^{\nu })={\frac {\lambda ^{\nu }\Gamma (\alpha -\nu )\Gamma (1+\nu )}{\Gamma (\alpha )}}}$

### Relation to the Pareto distribution

The Lomax distribution is a Pareto Type I distribution shifted so that its support begins at zero. Specifically:

${\displaystyle {\text{If }}Y\sim {\mbox{Pareto}}(x_{m}=\lambda ,\alpha ),{\text{ then }}Y-x_{m}\sim {\mbox{Lomax}}(\alpha ,\lambda ).}$

The Lomax distribution is a Pareto Type II distribution with xm=λ and μ=0:[5]

${\displaystyle {\text{If }}X\sim {\mbox{Lomax}}(\alpha ,\lambda ){\text{ then }}X\sim {\text{P(II)}}(x_{m}=\lambda ,\alpha ,\mu =0).}$

### Relation to the generalized Pareto distribution

The Lomax distribution is a special case of the generalized Pareto distribution. Specifically:

${\displaystyle \mu =0,~\xi ={1 \over \alpha },~\sigma ={\lambda \over \alpha }.}$

### Relation to the beta prime distribution

The Lomax distribution with scale parameter λ = 1 is a special case of the beta prime distribution. If X has a Lomax distribution, then ${\displaystyle {\frac {X}{\lambda }}\sim \beta ^{\prime }(1,\alpha )}$ .

### Relation to the F distribution

The Lomax distribution with shape parameter α = 1 and scale parameter λ = 1 has density ${\displaystyle f(x)={\frac {1}{(1+x)^{2}}}}$ , the same distribution as an F(2,2) distribution. This is the distribution of the ratio of two independent and identically distributed random variables with exponential distributions.

### Relation to the q-exponential distribution

The Lomax distribution is a special case of the q-exponential distribution. The q-exponential extends this distribution to support on a bounded interval. The Lomax parameters are given by:

${\displaystyle \alpha ={{2-q} \over {q-1}},~\lambda ={1 \over \lambda _{q}(q-1)}.}$

### Relation to the (log-) logistic distribution

The logarithm of a Lomax(shape=1.0, scale=λ)-distributed variable follows a logistic distribution with location log(λ) and scale 1.0. This implies that a Lomax(shape=1.0, scale=λ)-distribution equals a log-logistic distribution with shape β=1.0 and scale α=log(λ).

### Gamma-exponential mixture connection

The Lomax distribution arises as a mixture of exponential distributions where the mixing distribution of the rate is a gamma distribution. If λ|k,θ ~ Gamma(shape=k, scale=θ) and X|λ ~ Exponential(rate=λ) then the marginal distribution of X|k,θ is Lomax(shape=k, scale=1/θ).

## References

1. Lomax, K. S. (1954) "Business Failures; Another example of the analysis of failure data". Journal of the American Statistical Association, 49, 847–852. JSTOR 2281544
2. Johnson, N. L.; Kotz, S.; Balakrishnan, N. (1994). "20 Pareto distributions". Continuous univariate distributions. 1 (2nd ed.). New York: Wiley. p. 573.
3. J. Chen, J., Addie, R. G., Zukerman. M., Neame, T. D. (2015) "Performance Evaluation of a Queue Fed by a Poisson Lomax Burst Process", IEEE Communications Letters, 19, 3, 367-370.
4. Van Hauwermeiren M and Vose D (2009). A Compendium of Distributions [ebook]. Vose Software, Ghent, Belgium. Available at www.vosesoftware.com.
5. Kleiber, Christian; Kotz, Samuel (2003), Statistical Size Distributions in Economics and Actuarial Sciences, Wiley Series in Probability and Statistics, 470, John Wiley & Sons, p. 60, ISBN 9780471457169.
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