Log-normal distribution

Log-normal
Probability density function


Some log-normal density functions with identical location parameter \mu but differing scale parameters \sigma

Cumulative distribution function


Cumulative distribution function of the log-normal distribution (with \mu = 0 )

Notation \ln\mathcal{N}(\mu,\,\sigma^2)
Parameters \mu \in \mathbb R — location,
\sigma > 0 — scale
of associated normal
Support x \in (0, +\infty)
PDF \frac{1}{x\sigma\sqrt{2\pi}}\ e^{-\frac{\left(\ln x-\mu\right)^2}{2\sigma^2}}
CDF \frac12 + \frac12\,\mathrm{erf}\Big[\frac{\ln x-\mu}{\sqrt{2}\sigma}\Big]
Mean e^{\mu+\sigma^2/2}
Median e^{\mu}\,
Mode e^{\mu-\sigma^2}
Variance (e^{\sigma^2}\!\!-1) e^{2\mu+\sigma^2}
Skewness (e^{\sigma^2}\!\!+2) \sqrt{e^{\sigma^2}\!\!-1}
Ex. kurtosis e^{4\sigma^2}\!\! + 2e^{3\sigma^2}\!\! + 3e^{2\sigma^2}\!\! - 6
Entropy \log(\sigma e^{\mu+\tfrac12} \sqrt{2\pi} )
MGF defined only on the negative half-axis, see text
CF representation \sum_{n=0}^{\infty}\frac{(it)^n}{n!}e^{n\mu+n^2\sigma^2/2} is asymptotically divergent but sufficient for numerical purposes
Fisher information \begin{pmatrix}1/\sigma^2&0\\0&2/\sigma^2\end{pmatrix}

In probability theory, a log-normal (or lognormal) distribution is a continuous probability distribution of a random variable whose logarithm is normally distributed. Thus, if the random variable X is log-normally distributed, then Y = \ln(X) has a normal distribution. Likewise, if Y has a normal distribution, then X = \exp(Y) has a log-normal distribution. A random variable which is log-normally distributed takes only positive real values. The distribution is occasionally referred to as the Galton distribution or Galton's distribution, after Francis Galton.[1] The log-normal distribution also has been associated with other names, such as McAlister, Gibrat and Cobb–Douglas.[1]

A log-normal process is the statistical realization of the multiplicative product of many independent random variables, each of which is positive. This is justified by considering the central limit theorem in the log domain. The log-normal distribution is the maximum entropy probability distribution for a random variate X for which the mean and variance of \ln(X) are specified.[2]

Notation

Given a log-normally distributed random variable X and two parameters \mu and \sigma that are, respectively, the mean and standard deviation of the variable’s natural logarithm, then the logarithm of X is normally distributed, and we can write X as

 X=e^{\mu+\sigma Z}

with Z a standard normal variable.

This relationship is true regardless of the base of the logarithmic or exponential function. If \log_a(Y) is normally distributed, then so is \log_b(Y), for any two positive numbers a,b\neq 1. Likewise, if e^X is log-normally distributed, then so is a^X, where a is a positive number \neq 1.

On a logarithmic scale, \mu and \sigma can be called the location parameter and the scale parameter, respectively.

In contrast, the mean, standard deviation, and variance of the non-logarithmized sample values are respectively denoted m, s.d., and v in this article. The two sets of parameters can be related as (see also Arithmetic moments below)[3]


\mu=\ln\left(\frac{m}{\sqrt{1+\frac{v}{m^2}}}\right), \sigma=\sqrt{\ln\left(1+\frac{v}{m^2}\right)}.

Characterization

Probability density function

A random positive variable x is log-normally distributed if the logarithm of x is normally distributed,


 \mathcal{N}(\mbox{ln}x;\mu,\sigma) = \frac{1}{\sigma\sqrt{2\pi}} \exp\left[-\frac {(\mbox{ln}x - \mu)^{2}} {2\sigma^{2}}\right], \ \ x>0.

A change of variables must conserve differential probability. In particular,


 \mathcal{N}(\mbox{ln}x)d\mbox{ln}x =  \mathcal{N}(\mbox{ln}x)\frac{d\mbox{ln}x}{dx}dx =  \mathcal{N}(\mbox{ln}x)\frac{dx}{x} = {\ln\mathcal{N}}(x) dx,

where

{\ln\mathcal{N}}(x;\mu,\sigma) = \frac{1}{ x\sigma \sqrt{2 \pi}}\exp\left[-\frac {(\mbox{ln}x - \mu)^{2}} {2\sigma^{2}}\right],\ \ x>0

is the log-normal probability density function.[1]

Cumulative distribution function

The cumulative distribution function is

\int_{0}^{x} {\ln\mathcal{N}}(\xi;\mu,\sigma) \, d\xi = \frac12 \left[ 1 + \operatorname{erf} \left(\frac{\ln x - \mu}{\sigma\sqrt{2}}\right) \right] = \frac12 \operatorname{erfc} \left(-\frac{\ln x - \mu}{\sigma\sqrt{2}}\right) = \Phi\left(\frac{\ln x - \mu}{\sigma}\right),

where erfc is the complementary error function, and Φ is the cumulative distribution function of the standard normal distribution.

Characteristic function and moment generating function

All moments of the log-normal distribution exist and it holds that: \operatorname{E}[X^n]=\mathrm{e}^{n\mu+\frac{n^2\sigma^2}{2}} (which can be derived by letting z=\frac{\ln(x) - (\mu+n\sigma^2)}{\sigma} within the integral). However, the expected value \operatorname{E}[e^{t X}] is not defined for any positive value of the argument t as the defining integral diverges. In consequence the moment generating function is not defined.[4] The last is related to the fact that the lognormal distribution is not uniquely determined by its moments.

Similarly, the characteristic function \operatorname{E}[e^{i t X}] is not defined in the half complex plane and therefore it is not analytic in the origin. In consequence, the characteristic function of the log-normal distribution cannot be represented as an infinite convergent series.[5] In particular, its Taylor formal series \sum_{n=0}^\infty \frac{(it)^n}{n!}e^{n\mu+n^2\sigma^2/2} diverges. However, a number of alternative divergent series representations have been obtained[5][6][7][8]

A closed-form formula for the characteristic function \varphi(t) with t in the domain of convergence is not known. A relatively simple approximating formula is available in closed form and given by[9]

\varphi(t)\approx\frac{\exp\left(-\frac{W^2(t\sigma^2e^\mu)+2W(t\sigma^2e^\mu)}{2\sigma^2} \right)}{\sqrt{1+W(t\sigma^2e^\mu)}}

where W is the Lambert W function. This approximation is derived via an asymptotic method but it stays sharp all over the domain of convergence of \varphi.

Properties

Location and scale

The location and scale parameters of a log-normal distribution, i.e. \mu and \sigma, are more readily treated using the geometric mean, \mathrm{GM}[X], and the geometric standard deviation, \mathrm{GSD}[X], rather than the arithmetic mean, \mathrm{E}[X], and the arithmetic standard deviation, \mathrm{SD}[X].

Geometric moments

The geometric mean of the log-normal distribution is \mathrm{GM}[X] = e^{\mu}, and the geometric standard deviation is \mathrm{GSD}[X] = e^{\sigma}.[10][11] By analogy with the arithmetic statistics, one can define a geometric variance, \mathrm{GVar}[X] = e^{\sigma^2}, and a geometric coefficient of variation,[10] \mathrm{GCV}[X] = e^{\sigma} - 1.

Because the log-transformed variable Y = \ln X is symmetric and quantiles are preserved under monotonic transformations, the geometric mean of a log-normal distribution is equal to its median, \mathrm{Med}[X].[12]

Note that the geometric mean is less than the arithmetic mean. This is due to the AM–GM inequality, and corresponds to the logarithm being convex down. In fact,


\begin{align}
  \mathrm{E}[X] &= e^{\mu + \frac12 \sigma^2} &= e^{\mu} \cdot \sqrt{e^{\sigma^2}} &= \mathrm{GM}[X] \cdot \sqrt{\mathrm{GVar}[X]}.
\end{align}

In finance the term e^{-\frac12\sigma^2} is sometimes interpreted as a convexity correction. From the point of view of stochastic calculus, this is the same correction term as in Itō's lemma for geometric Brownian motion.

Arithmetic moments

For any real or complex number s, the s-th moment of a log-normally distributed variable X is given by[1]

\operatorname{E}[X^s] = e^{s\mu + \frac12s^2\sigma^2}.

Specifically, the arithmetic mean, expected square, arithmetic variance, and arithmetic standard deviation of a log-normally distributed variable X are given by

\begin{align}
  & \operatorname{E}[X] = e^{\mu + \tfrac{1}{2}\sigma^2}, \\
  & \operatorname{E}[X^2] = e^{2\mu + 2\sigma^2}, \\
  & \operatorname{Var}[X] = \operatorname{E}[X^2] - \operatorname{E}[X]^2 = (\operatorname{E}[X])^2(e^{\sigma^2} - 1) = e^{2\mu + \sigma^2} (e^{\sigma^2} - 1), \\
  & \operatorname{SD}[X] = \sqrt{\operatorname{Var}[X]} = e^{\mu + \tfrac{1}{2}\sigma^2}\sqrt{e^{\sigma^2} - 1}
  = \operatorname{E}[X] \sqrt{e^{\sigma^2} - 1},
  \end{align}

respectively.

The location (μ) and scale (σ) parameters can be obtained if the arithmetic mean and the arithmetic variance are known:

\begin{align}
  \mu &= \ln \left(\frac{\operatorname{E}[X]^2}{\sqrt{\operatorname{E}[X^2]}}\right) = \ln \left( \frac{\operatorname{E}[X]^2}{\sqrt{\operatorname{Var}[X] + \operatorname{E}[X]^2}} \right), \\
  \sigma^2 &=  \ln \left(\frac{\operatorname{E}[X^2]}{\operatorname{E}[X]^2}\right) =  \ln \left(1 + \frac{\operatorname{Var}[X]}{\operatorname{E}[X]^2}\right).
  \end{align}

A log-normal distribution is not uniquely determined by its moments E[Xk] for k ≥ 1. That is, there exists some other distribution with the same moments for all k.[1] In fact, there is a whole family of distributions with the same moments as the log-normal distribution.

Mode and median

Comparison of mean, median and mode of two log-normal distributions with different skewness.

The mode is the point of global maximum of the probability density function. In particular, it solves the equation (\ln f)'=0:

\mathrm{Mode}[X] = e^{\mu - \sigma^2}.

The median is such a point where F_X=0.5:

\mathrm{Med}[X] = e^\mu\,.

Arithmetic coefficient of variation

The arithmetic coefficient of variation \mathrm{CV}[X] is the ratio \frac{\mathrm{SD}[X]}{\mathrm{E}[X]} (on the natural scale). For a log-normal distribution it is equal to

\mathrm{CV}[X] = \sqrt{e^{\sigma^2} - 1}.

Contrary to the arithmetic standard deviation, the arithmetic coefficient of variation is independent of the arithmetic mean.

Partial expectation

The partial expectation of a random variable X with respect to a threshold k is defined as  g(k) = \int_k^\infty x \ln \mathcal{N} (x)\, dx where  \ln\mathcal{N}(x) is the probability density function of X. Alternatively, and using the definition of conditional expectation, it can be written as g(k)=\operatorname{E}[X\mid X>k] P(X>k). For a log-normal random variable the partial expectation is given by:

g(k) = \int_k^\infty x \ln\mathcal{N}(x)\, dx
            = e^{\mu+\tfrac{1}{2}\sigma^2}\, \Phi\!\left(\frac{\mu+\sigma^2-\ln k}{\sigma}\right)

where Φ is the normal cumulative distribution function. The derivation of the formula is provided in the discussion of this Wikipedia entry. The partial expectation formula has applications in insurance and economics, it is used in solving the partial differential equation leading to the Black–Scholes formula.

Conditional expectation

The conditional expectation of a lognormal random variable X with respect to a threshold k is its partial expectation divided by the cumulative probability of being in that range:

E[X\mid X<k]=e^{\mu +\sigma^2/2}\cdot \frac{\Phi \left[\frac{\ln(k)-\mu -\sigma^2}{\sigma} \right]}{\frac{1}{2}+\frac{1}{2} \operatorname{erf}\left[\frac{\ln(k)-\mu }{\sqrt{2}\sigma} \right]}
E[X\mid X\geqslant k]=e^{\mu +\sigma^2/2}\cdot \frac{\Phi \left[\frac{\mu +\sigma^2-\ln(k)}{\sigma} \right]}{\frac{1}{2}-\frac{1}{2} \operatorname{erf}\left[\frac{\ln(k)-\mu }{\sqrt{2}\sigma} \right]}

Other

A set of data that arises from the log-normal distribution has a symmetric Lorenz curve (see also Lorenz asymmetry coefficient).[13]

The harmonic H, geometric G and arithmetic A means of this distribution are related;[14] such relation is given by

H = \frac{G^2}{ A} .

Log-normal distributions are infinitely divisible,[15] but they are not stable distributions, which can be easily drawn from.[16]

Occurrence

The log-normal distribution is important in the description of natural phenomena. This follows, because many natural growth processes are driven by the accumulation of many small percentage changes. These become additive on a log scale. If the effect of any one change is negligible, the central limit theorem says that the distribution of their sum is more nearly normal than that of the summands. When back-transformed onto the original scale, it makes the distribution of sizes approximately log-normal (though if the standard deviation is sufficiently small, the normal distribution can be an adequate approximation).

This multiplicative version of the central limit theorem is also known as Gibrat's law, after Robert Gibrat (1904–1980) who formulated it for companies.[17] If the rate of accumulation of these small changes does not vary over time, growth becomes independent of size. Even if that's not true, the size distributions at any age of things that grow over time tends to be log-normal.

Examples include the following:

Fitted cumulative log-normal distribution to annually maximum 1-day rainfalls, see distribution fitting
Consequently, reference ranges for measurements in healthy individuals are more accurately estimated by assuming a log-normal distribution than by assuming a symmetric distribution about the mean.

Maximum likelihood estimation of parameters

For determining the maximum likelihood estimators of the log-normal distribution parameters μ and σ, we can use the same procedure as for the normal distribution. To avoid repetition, we observe that

L (x;\mu, \sigma) = \prod_{i=1}^n \left(\frac 1 x_i\right) \, \mathcal{N} (\ln x; \mu, \sigma)

where by L we denote the probability density function of the log-normal distribution and by \mathcal{N} that of the normal distribution. Therefore, using the same indices to denote distributions, we can write the log-likelihood function thus:


\begin{align}
\ell_L (\mu,\sigma \mid x_1, x_2, \dots, x_n)
  & {} = - \sum _k \ln x_k + \ell_N (\mu, \sigma \mid \ln x_1, \ln x_2, \dots, \ln x_n) \\
& {} = \text{constant} + \ell_N (\mu, \sigma \mid \ln x_1, \ln x_2, \dots, \ln x_n).
\end{align}

Since the first term is constant with regard to μ and σ, both logarithmic likelihood functions, \ell_L and \ell_N, reach their maximum with the same \mu and \sigma. Hence, using the formulas for the normal distribution maximum likelihood parameter estimators and the equality above, we deduce that for the log-normal distribution it holds that

\widehat \mu = \frac {\sum_k \ln x_k} n,
        \widehat \sigma^2 = \frac {\sum_k \left( \ln x_k - \widehat \mu \right)^2} {n}.

Multivariate log-normal

If \boldsymbol X \sim \mathcal{N}(\boldsymbol\mu,\,\boldsymbol\Sigma) is a multivariate normal distribution then \boldsymbol Y=\exp(\boldsymbol X) has a multivariate log-normal distribution[35] with mean

\operatorname{E}[\boldsymbol Y]_i=e^{\mu_i+\frac{1}{2}\Sigma_{ii}} ,

and covariance matrix

\operatorname{Var}[\boldsymbol Y]_{ij}=e^{\mu_i+\mu_j + \frac{1}{2}(\Sigma_{ii}+\Sigma_{jj}) }( e^{\Sigma_{ij}} - 1) .

Related distributions

Y \sim \operatorname{\ln\mathcal{N}}\Big(\textstyle \sum_{j=1}^n\mu_j,\ \sum_{j=1}^n \sigma_j^2 \Big).
\begin{align}
  \sigma^2_Z &= \ln\!\left[ \frac{\sum e^{2\mu_j+\sigma_j^2}(e^{\sigma_j^2}-1)}{(\sum e^{\mu_j+\sigma_j^2/2})^2} + 1\right], \\
  \mu_Z &= \ln\!\left[ \sum e^{\mu_j+\sigma_j^2/2} \right] - \frac{\sigma^2_Z}{2}.
  \end{align}

In the case that all X_j have the same variance parameter \sigma_j=\sigma, these formulas simplify to

\begin{align}
  \sigma^2_Z &= \ln\!\left[ (e^{\sigma^2}-1)\frac{\sum e^{2\mu_j}}{(\sum e^{\mu_j})^2} + 1\right], \\
  \mu_Z &= \ln\!\left[ \sum e^{\mu_j} \right] + \frac{\sigma^2}{2} -  \frac{\sigma^2_Z}{2}.
  \end{align}

Similar distributions

A substitute for the log-normal whose integral can be expressed in terms of more elementary functions[38] can be obtained based on the logistic distribution to get an approximation for the CDF

 F(x;\mu,\sigma) = \left[\left(\frac{e^\mu}{x}\right)^{\pi/(\sigma \sqrt{3})} +1\right]^{-1}.

This is a log-logistic distribution.

See also

Notes

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  2. Park, Sung Y.; Bera, Anil K. (2009). "Maximum entropy autoregressive conditional heteroskedasticity model" (PDF). Journal of Econometrics (Elsevier) 150 (2): 219–230. doi:10.1016/j.jeconom.2008.12.014. Retrieved 2011-06-02.
  3. "Lognormal mean and variance"
  4. Heyde, CC. (1963), "On a property of the lognormal distribution", Journal of the Royal Statistical Society, Series B (Methodological) 25 (2): 392–393, doi:10.1007/978-1-4419-5823-5_6
  5. 1 2 Holgate, P. (1989). "The lognormal characteristic function, vol. 18, pp. 4539–4548, 1989". Communications in Statistical – Theory and Methods 18 (12): 4539–4548. doi:10.1080/03610928908830173.
  6. Barakat, R. (1976). "Sums of independent lognormally distributed random variables". Journal of the Optical Society of America 66 (3): 211–216. doi:10.1364/JOSA.66.000211.
  7. Barouch, E.; Kaufman, GM.; Glasser, ML. (1986). "On sums of lognormal random variables" (PDF). Studies in Applied Mathematics 75 (1): 37–55.
  8. Leipnik, Roy B. (January 1991). "On Lognormal Random Variables: I – The Characteristic Function". Journal of the Australian Mathematical Society Series B 32 (3): 327–347. doi:10.1017/S0334270000006901.
  9. S. Asmussen, J.L. Jensen, L. Rojas-Nandayapa. "On the Laplace transform of the Lognormal distribution", Thiele centre preprint, (2013).
  10. 1 2 Kirkwood, Thomas BL (Dec 1979). "Geometric means and measures of dispersion". Biometrics 35 (4): 908–9. doi:10.2307/2530139.
  11. Limpert, E; Stahel, W; Abbt, M (2001). "Lognormal distributions across the sciences: keys and clues". BioScience 51 (5): 341–352. doi:10.1641/0006-3568(2001)051[0341:LNDATS]2.0.CO;2.
  12. Daly, Leslie E.; Bourke, Geoffrey Joseph (2000). Interpretation and uses of medical statistics (5th ed.). Wiley-Blackwell. p. 89. doi:10.1002/9780470696750. ISBN 978-0-632-04763-5.
  13. Damgaard, Christian; Weiner, Jacob (2000). "Describing inequality in plant size or fecundity". Ecology 81 (4): 1139–1142. doi:10.1890/0012-9658(2000)081[1139:DIIPSO]2.0.CO;2.
  14. Rossman, Lewis A (July 1990). "Design stream flows based on harmonic means". J Hydraulic Engineering 116 (7): 946–950. doi:10.1061/(ASCE)0733-9429(1990)116:7(946).
  15. Thorin, Olof (1977). "On the infinite divisibility of the lognormal distribution". Scandinavian Actuarial Journal 1977 (3): 121–148. doi:10.1080/03461238.1977.10405635. ISSN 0346-1238.
  16. 1 2 Gao, X.; Xu, H; Ye, D. (2009), "Asymptotic Behaviors of Tail Density for Sum of Correlated Lognormal Variables". International Journal of Mathematics and Mathematical Sciences, vol. 2009, Article ID 630857. doi:10.1155/2009/630857
  17. Sutton, John (Mar 1997). "Gibrat's Legacy". Journal of Economic Literature 32 (1): 40–59. JSTOR 2729692.
  18. Pawel, Sobkowicz; et al. (2013). "Lognormal distributions of user post lengths in Internet discussions - a consequence of the Weber-Fechner law?". EPJ Data Science.
  19. Yin, Peifeng; Luo, Ping; Lee, Wang-Chien; Wang, Min (2013). Silence is also evidence: interpreting dwell time for recommendation from psychological perspective. ACM International Conference on KDD.
  20. Huxley, Julian S. (1932). Problems of relative growth. London. ISBN 0-486-61114-0. OCLC 476909537.
  21. 1 2 Wang, WenBin; Wu, ZiNiu; Wang, ChunFeng; Hu, RuiFeng (2013). "Modelling the spreading rate of controlled communicable epidemics through an entropy-based thermodynamic model". Science China Physics, Mechanics and Astronomy 56 (11): 2143–2150. doi:10.1007/s11433-013-5321-0. ISSN 1674-7348.
  22. Makuch, Robert W.; D.H. Freeman; M.F. Johnson (1979). "Justification for the lognormal distribution as a model for blood pressure". Journal of Chronic Diseases 32 (3): 245–250. doi:10.1016/0021-9681(79)90070-5. Retrieved 27 February 2012.
  23. Ritzema (ed.), H.P. (1994). Frequency and Regression Analysis (PDF). Chapter 6 in: Drainage Principles and Applications, Publication 16, International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. pp. 175–224. ISBN 90-70754-33-9.
  24. Clementi, Fabio; Gallegati, Mauro (2005) "Pareto's law of income distribution: Evidence for Germany, the United Kingdom, and the United States", EconWPA
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  30. http://wireless.per.nl/reference/chaptr03/shadow/shadow.htm Archived May 9, 2015, at the Wayback Machine.
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  34. http://chess.stackexchange.com/questions/2506/what-is-the-average-length-of-a-game-of-chess/4899#4899
  35. Tarmast, Ghasem (2001). Multivariate Log–Normal Distribution (PDF). ISI Proceedings: 53rd Session. Seoul.
  36. Asmussen, S.; Rojas-Nandayapa, L. (2008). "Asymptotics of Sums of Lognormal Random Variables with Gaussian Copula". Statistics and Probability Letters 78 (16): 2709–2714. doi:10.1016/j.spl.2008.03.035.
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References

Further reading

External links

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