L-moment
In statistics, L-moments are a sequence of statistics used to summarize the shape of a probability distribution.[1][2][3][4] They are linear combinations of order statistics (L-statistics) analogous to conventional moments, and can be used to calculate quantities analogous to standard deviation, skewness and kurtosis, termed the L-scale, L-skewness and L-kurtosis respectively (the L-mean is identical to the conventional mean). Standardised L-moments are called L-moment ratios and are analogous to standardized moments. Just as for conventional moments, a theoretical distribution has a set of population L-moments. Sample L-moments can be defined for a sample from the population, and can be used as estimators of the population L-moments.
Population L-moments
For a random variable X, the rth population L-moment is[1]
where Xk:n denotes the rth
Note that the coefficients of the rth L-moment are the same as in the rth term of the binomial transform, as used in the r-order finite difference (finite analog to the derivative).
The first two of these L-moments have conventional names:
- is the "mean", "L-mean", or "L-location",
- is the "L-scale".
The L-scale is equal to half the Mean absolute difference.[5]
Sample L-moments
The sample L-moments can be computed as the population L-moments of the sample, summing over r-element subsets of the sample hence averaging by dividing by the binomial coefficient:
Grouping these by order statistic counts the number of ways an element of an n element sample can be the jth element of an r element subset, and yields formulas of the form below. Direct estimators for the first four L-moments in a finite sample of n observations are:[6]
where x(i) is the ith order statistic and is a binomial coefficient. Sample L-moments can also be defined indirectly in terms of probability weighted moments,[1][7][8] which leads to a more efficient algorithm for their computation.[6][9]
L-moment ratios
A set of L-moment ratios, or scaled L-moments, is defined by
The most useful of these are called the L-skewness, and the L-kurtosis.
L-moment ratios lie within the interval ( −1, 1 ) . Tighter bounds can be found for some specific L-moment ratios; in particular, the L-kurtosis lies in [ −+ 1 /4, 1 ) , and
A quantity analogous to the coefficient of variation, but based on L-moments, can also be defined: which is called the "coefficient of L-variation", or "L-CV". For a non-negative random variable, this lies in the interval ( 0, 1 ) [1] and is identical to the Gini coefficient.[10]
Related quantities
L-moments are statistical quantities that are derived from probability weighted moments[11] (PWM) which were defined earlier (1979).[7] PWM are used to efficiently estimate the parameters of distributions expressable in inverse form such as the Gumbel,[8] the Tukey lambda, and the Wakeby distributions.
Usage
There are two common ways that L-moments are used, in both cases analogously to the conventional moments:
- As summary statistics for data.
- To derive estimators for the parameters of probability distributions, applying the method of momentsto the L-moments rather than conventional moments.
In addition to doing these with standard moments, the latter (estimation) is more commonly done using
As an example consider a dataset with a few data points and one outlying data value. If the ordinary
Another advantage L-moments have over conventional moments is that their existence only requires the random variable to have finite mean, so the L-moments exist even if the higher conventional moments do not exist (for example, for
Some appearances of L-moments in the statistical literature include the book by David & Nagaraja (2003, Section 9.9)[12] and a number of papers.[10][13][14][15][16][17] A number of favourable comparisons of L-moments with ordinary moments have been reported.[18][19]
Values for some common distributions
The table below gives expressions for the first two L moments and numerical values of the first two L-moment ratios of some common
More complex expressions have been derived for some further distributions for which the L-moment ratios vary with one or more of the distributional parameters, including the log-normal, Gamma, generalized Pareto, generalized extreme value, and generalized logistic distributions.[1]Distribution Parameters mean, λ1 L-scale, λ2 L-skewness, τ3 L-kurtosis, τ4 Uniform a, b 1 /2(a + b) 1 /6(b – a) 0 0 Logistic μ, s μ s 0 1 /6 = 0.1667 Normal μ, σ2 μ σ/ √π 0 30 θm/π - 9 = 0.1226 Laplace μ, b μ 3 / 4 b 0 1/ 3 √2 = 0.2357 Student's t, 2 d.f. ν = 2 0 π/ 2 √2 = 1.111 0 3 / 8 = 0.375 Student's t, 4 d.f. ν = 4 0 15 /64 π = 0.7363 0 111 /512 = 0.2168 Exponential λ 1/ λ 1/ 2 λ 1 /3 = 0.3333 1 /6 = 0.1667 Gumbel μ, β μ + γeββ log2(3) 2 log2(3) - 3 = 0.1699 16 - 10 log2(3) = 0.1504
The notation for the parameters of each distribution is the same as that used in the linked article. In the expression for the mean of the
Extensions
Trimmed L-moments are generalizations of L-moments that give zero weight to extreme observations. They are therefore more robust to the presence of outliers, and unlike L-moments they may be well-defined for distributions for which the mean does not exist, such as the Cauchy distribution.[20]
See also
References
- ^ JSTOR 2345653.
- JSTOR 2685210.
- .
- ISBN 1-463-50841-7
- ^ a b
Jones, M.C. (2002). "Student's simplest distribution". JSTOR 3650389.
- ^ a b Wang, Q.J. (1996). "Direct sample estimators of L-moments". Water Resources Research. 32 (12): 3617–3619. .
- ^ a b
Greenwood, J.A.; Landwehr, J.M.; Matalas, N.C.; Wallis, J.R. (1979). "Probability weighted moments: Definition and relation to parameters of several distributions expressed in inverse form" (PDF). Water Resources Research. 15 (5): 1049–1054. S2CID 121955257. Archived from the original(PDF) on 2020-02-10.
- ^ a b Landwehr, J.M.; Matalas, N.C.; Wallis, J.R. (1979). "Probability weighted moments compared with some traditional techniques in estimating Gumbel parameters and quantiles". Water Resources Research. 15 (5): 1055–1064. .
- ^ "L moments". NIST Dataplot. itl.nist.gov (documentation). National Institute of Standards and Technology. 6 January 2006. Retrieved 19 January 2013.
- ^ a b Valbuena, R.; Maltamo, M.; Mehtätalo, L.; Packalen, P. (2017). "Key structural features of Boreal forests may be detected directly using L-moments from airborne lidar data". Remote Sensing of Environment. 194: 437–446. .
- ISBN 978-0521019408. Retrieved 22 January 2013.
- ISBN 978-0-471-38926-2.
- .
- .
- .
- .
- .
- PMID 1609174.
- S2CID 120542594.
- .
External links
- The L-moments page Jonathan R.M. Hosking, IBM Research
- L Moments. Dataplot reference manual, vol. 1, auxiliary chapter. National Institute of Standards and Technology, 2006. Accessed 2010-05-25.
- Lmo lightweight Python includes functions for fast calculation of L-moments, trimmed L-moments, and multivariate L-comoments.