[section:skew_normal_dist Skew Normal Distribution] ``#include `` namespace boost{ namespace math{ template class skew_normal_distribution; typedef skew_normal_distribution<> normal; template class skew_normal_distribution { public: typedef RealType value_type; typedef Policy policy_type; // Constructor: skew_normal_distribution(RealType location = 0, RealType scale = 1, RealType shape = 0); // Accessors: RealType location()const; // mean if normal. RealType scale()const; // width, standard deviation if normal. RealType shape()const; // The distribution is right skewed if shape > 0 and is left skewed if shape < 0. // The distribution is normal if shape is zero. }; }} // namespaces The skew normal distribution is a variant of the most well known Gaussian statistical distribution. The skew normal distribution with shape zero resembles the [@http://en.wikipedia.org/wiki/Normal_distribution Normal Distribution], hence the latter can be regarded as a special case of the more generic skew normal distribution. If the standard (mean = 0, scale = 1) normal distribution probability density function is [equation normal01_pdf] and the cumulative distribution function [equation normal01_cdf] then the [@http://en.wikipedia.org/wiki/Probability_density_function PDF] of the [@http://en.wikipedia.org/wiki/Skew_normal_distribution skew normal distribution] with shape parameter [alpha], defined by O'Hagan and Leonhard (1976) is [equation skew_normal_pdf0] Given [@http://en.wikipedia.org/wiki/Location_parameter location] [xi], [@http://en.wikipedia.org/wiki/Scale_parameter scale] [omega], and [@http://en.wikipedia.org/wiki/Shape_parameter shape] [alpha], it can be [@http://en.wikipedia.org/wiki/Skew_normal_distribution transformed], to the form: [equation skew_normal_pdf] and [@http://en.wikipedia.org/wiki/Cumulative_distribution_function CDF]: [equation skew_normal_cdf] where ['T(h,a)] is Owen's T function, and ['[Phi](x)] is the normal distribution. The variation the PDF and CDF with its parameters is illustrated in the following graphs: [graph skew_normal_pdf] [graph skew_normal_cdf] [h4 Member Functions] skew_normal_distribution(RealType location = 0, RealType scale = 1, RealType shape = 0); Constructs a skew_normal distribution with location [xi], scale [omega] and shape [alpha]. Requires scale > 0, otherwise __domain_error is called. RealType location()const; returns the location [xi] of this distribution, RealType scale()const; returns the scale [omega] of this distribution, RealType shape()const; returns the shape [alpha] of this distribution. (Location and scale function match other similar distributions, allowing the functions `find_location` and `find_scale` to be used generically). [note While the shape parameter may be chosen arbitrarily (finite), the resulting [*skewness] of the distribution is in fact limited to about (-1, 1); strictly, the interval is (-0.9952717, 0.9952717). A parameter [delta] is related to the shape [alpha] by [delta] = [alpha] / (1 + [alpha][pow2]), and used in the expression for skewness [equation skew_normal_skewness] ] [/note] [h4 References] * [@http://azzalini.stat.unipd.it/SN/ Skew-Normal Probability Distribution] for many links and bibliography. * [@http://azzalini.stat.unipd.it/SN/Intro/intro.html A very brief introduction to the skew-normal distribution] by Adelchi Azzalini (2005-11-2). * See a [@http://www.tri.org.au/azzalini.html skew-normal function animation]. [h4 Non-member Accessors] All the [link math_toolkit.dist_ref.nmp usual non-member accessor functions] that are generic to all distributions are supported: __usual_accessors. The domain of the random variable is ['-[max_value], +[min_value]]. Infinite values are not supported. There are no [@http://en.wikipedia.org/wiki/Closed-form_expression closed-form expression] known for the mode and median, but these are computed for the * mode - by finding the maximum of the PDF. * median - by computing `quantile(1/2)`. The maximum of the PDF is sought through searching the root of f'(x)=0. Both involve iterative methods that will have lower accuracy than other estimates. [h4 Testing] __R using library(sn) described at [@http://azzalini.stat.unipd.it/SN/ Skew-Normal Probability Distribution], and at [@http://cran.r-project.org/web/packages/sn/sn.pd R skew-normal(sn) package]. Package sn provides functions related to the skew-normal (SN) and the skew-t (ST) probability distributions, both for the univariate and for the the multivariate case, including regression models. __Mathematica was also used to generate some more accurate spot test data. [h4 Accuracy] The skew_normal distribution with shape = zero is implemented as a special case, equivalent to the normal distribution in terms of the [link math_toolkit.sf_erf.error_function error function], and therefore should have excellent accuracy. The PDF and mean, variance, skewness and kurtosis are also accurately evaluated using [@http://en.wikipedia.org/wiki/Analytical_expression analytical expressions]. The CDF requires [@http://en.wikipedia.org/wiki/Owen%27s_T_function Owen's T function] that is evaluated using a Boost C++ __owens_t implementation of the algorithms of M. Patefield and D. Tandy, Journal of Statistical Software, 5(5), 1-25 (2000); the complicated accuracy of this function is discussed in detail at __owens_t. The median and mode are calculated by iterative root finding, and both will be less accurate. [h4 Implementation] In the following table, [xi] is the location of the distribution, and [omega] is its scale, and [alpha] is its shape. [table [[Function][Implementation Notes]] [[pdf][Using:[equation skew_normal_pdf] ]] [[cdf][Using: [equation skew_normal_cdf][br] where ['T(h,a)] is Owen's T function, and ['[Phi](x)] is the normal distribution. ]] [[cdf complement][Using: complement of normal distribution + 2 * Owens_t]] [[quantile][Maximum of the pdf is sought through searching the root of f'(x)=0]] [[quantile from the complement][-quantile(SN(-location [xi], scale [omega], -shape[alpha]), p)]] [[location][location [xi]]] [[scale][scale [omega]]] [[shape][shape [alpha]]] [[median][quantile(1/2)]] [[mean][[equation skew_normal_mean]]] [[mode][Maximum of the pdf is sought through searching the root of f'(x)=0]] [[variance][[equation skew_normal_variance] ]] [[skewness][[equation skew_normal_skewness] ]] [[kurtosis][kurtosis excess-3]] [[kurtosis excess] [ [equation skew_normal_kurt_ex] ]] ] [/table] [endsect] [/section:skew_normal_dist skew_Normal] [/ skew_normal.qbk Copyright 2012 Benjamin Sobotta, John Maddock and Paul A. Bristow. Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt). ]