On The Exponential Power Distribution

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Mathematical Theory and ModelingISSN 2224-5804 (Paper)ISSN 2225-0522 (Online)Vol.8, No.8, 2018www.iiste.orgOn the Exponential Power DistributionBismark Kwao Nkansah1, Michael Manford2*, Nathaniel Haward11. Department of Statistics, University of Cape Coast, Cape Coast, Ghana2. Directorate of Academic Planning and Quality Assurance, Cape Coast Technical University, P. O. BoxAD50, Cape Coast, Ghana* E-mail of the corresponding author: mykemanford@yahoo.comAbstractThe paper examines the nature of the exponential power distribution (EPD) in terms of its location, Β΅, scale, Ξ²and shape, Οƒ, parameters. It establishes conditions under which the distribution is legitimate and reliable. Itderives among others the moment and kurtosis of the distribution as well as the maximum likelihood estimatorsof the parameters. It then uses data on health to assess the departure of the distribution from normality. Threemain softwares are used, namely; EasyFit, MATLAB and Minitab.In the application, we find that the EPD, for some values of 𝛽, significantly fits Weight, Height and Body MassIndex out of seven variables covered. We deduce that the EPD would be inappropriate for fitting asymmetricaldatasets, since the variables which are not significant are found to be highly skewed.Keywords: Exponential power distribution, Kurtosis, Legitimacy, Statistical Distributions1. IntroductionThe defining characteristics of statistical distributions are their dependence on parameters and the incorporationof stochastic terms. The properties of the distributions and the properties of quantities derived from them arestudied in a long-run, average sense through expectations, variances, skewness and kurtosis. The fact that theparameters of the distribution are estimated from the data introduces a stochastic element in applying a statisticaldistribution. This is because the distribution is not deterministic but includes randomness. Parameters and relatedquantities derived from the distribution are likewise random.A statistical distribution of a variable is an approximate representation of its population distribution which maybe parametric or non-parametric. A theoretical parametric distribution generally provides a simple parsimonious(and usually smooth) representation of the population distribution. It can be used for inference of the centiles (orquantiles) and moments of the population distribution and other population measures. Inference of the momentscan be particularly sensitive to misspecification of the theoretical distribution and especially to misspecificationof the heaviness of the tail(s) of the population distribution (Forbes, Evans, Hastings & Peacock, 2011).Over the last two decades, many researchers have developed interest in the construction of flexible parametricclasses of statistical distributions that are more flexible than the normal distribution. Many practical applicationsrequire models of data exhibiting a skewed or peaked distributions, and some researchers suggest the use ofdistributions which are more robust for such data. Some of these applications are in areas that include health,environmental and finance. There are several parametric classes of distributions to choose from. Rigby,Stasinopoulos, Heller and Bastiani (2017) have reviewed many of them. Subbotin (1923) introduced a class ofdistribution called the exponential power distribution (EPD) which is believed to be more flexible than thenormal distribution in terms of kurtosis.Subbotin in his study on the Law of Frequency of Error formulated an axiom which states that the probability ofa random error πœ€ depends only on the absolute value of the error itself and can be expressed by a function𝑓(πœ€) having continuous first derivative almost everywhere. Based on this axiom, Subbotin obtained a densityfunction called Subbotin’s family of distributions given by𝑓(πœ€) π‘šβ„Žπ‘’π‘₯𝑝{ β„Žπ‘š πœ€ π‘š },12Ξ“ ( )π‘š(1)with πœ€ , β„Ž 0 and π‘š 1. This class of distributions is said to be symmetric, but with variationin kurtosis. It was noted that this distribution has many structural properties close to the normal distribution.There is also a link in the axiom considered by Subbotin and that of Gauss, as Gauss used similar axiom to121

Mathematical Theory and ModelingISSN 2224-5804 (Paper)ISSN 2225-0522 (Online)Vol.8, No.8, 2018www.iiste.orgderive the usual normal distribution𝑓(π‘₯) 1𝜎 2πœ‹1 𝑦 πœ‡ 2𝑒π‘₯𝑝 { (2𝜎) },(2)with two parameters; πœ‡ , (mean or location parameter), and 𝜎 0 (standard deviation or scaleparameter). Several researchers (Coin, 2017; Giller, 2005; Nadarajah, 2005; PogΓ‘ny & Nadarajah, 2009; Tahir,Cordeiro, Alizadeh, Mansoor, Zubair & Hamedani, 2015) have introduced various classes of distribution relatingto the Subbotin’s family of distributions. Some studies have used the name the Generalized GaussianDistribution, Generalized Normal Distribution or Generalized Error Distribution to refer to the ExponentialPower Distribution.Giller (2005) in his study expressed the EPD as12π‘ž 1 𝜎 11 𝑦 πœ‡ π‘žπ‘ƒ(𝑦 πœ‡, 𝜎, π‘ž) 𝑒π‘₯𝑝 { },Ξ“(π‘ž 1)2 𝜎(3)Giller stated that if π‘ž 1/2, then 𝑃(𝑦 πœ‡, 𝜎, π‘ž) 𝑁(πœ‡, 𝜎 2 ) (Normal) and if π‘ž 1, then𝑃(𝑦 πœ‡, 𝜎, π‘ž) 𝐿(πœ‡, 4𝜎 2 ) (Double Exponential or Laplace). In the limit as π‘ž 0, 𝑃(𝑦 πœ‡, 𝜎, π‘ž) π‘ˆ(πœ‡ 𝜎, πœ‡ 𝜎)(Uniform).PogΓ‘ny and Nadarajah (2009) modified the EPD and expressed it as1π‘žπœŽ 1𝑦 πœ‡ π‘žπ‘ƒ(𝑦 πœ‡, 𝜎, π‘ž) 𝑒π‘₯𝑝 { }.1𝜎2Ξ“ ( )π‘ž(4)This distribution has three parameters given as πœ‡, 𝜎 and π‘ž which represent the location, scale (or dispersion)and shape of the distribution, respectively. They further noted that 𝑃(𝑦 πœ‡, 𝜎, 1) is Laplace (or DoubleExponential) and 𝑃(𝑦 πœ‡, 𝜎, 2) 𝑁(πœ‡, 𝜎 2 /2 ) . Also, the pointwise π‘™π‘–π‘šπ‘ž 𝑃(𝑦 πœ‡, 𝜎, π‘ž) coincides with thedensity function of uniform distribution, π‘ˆ(πœ‡ 𝜎, πœ‡ 𝜎).Mineo and Ruggieri (2005) expressed their EPD as𝑃(𝑦 πœ‡, 𝜎, π‘ž) π‘ž11 𝑦 πœ‡π‘’π‘₯𝑝 { },1π‘ž πœŽπ‘ž12qq Οƒπ‘ž Ξ“ (1 )π‘ž(5)Mineo and Ruggieri explained that the parameter q determines the shape of the curve; in this way, it is linked tothe thickness of the tails, and thus to the kurtosis, of the distribution. In fact, by changing the parameter q, theEPD describes both leptokurtic (0 π‘ž 2) and platikurtic (π‘ž 2) distributions.Purczyriski and Bednarz-Okrzyriska (2014) adopted a class of EPDs of the form𝑓(𝑦) πœ†π‘žπ‘’π‘₯𝑝{ 𝑦 πœ‡ π‘ž },12Ξ“ ( )π‘ž(6)where Ξ“(1/π‘ž) is an Euler’s gamma function. For π‘ž 1, the EPD turns into the Laplace distribution(bi-exponential), and for π‘ž 2, a Normal distribution is obtained given πœ† 1/𝜎 2.More generally, the EPD can be deduced as𝑃(𝑦 πœ‡, 𝜎, π‘ž) π‘˜πœŽ 1 𝑒π‘₯𝑝 { 𝑐 𝑦 πœ‡ π‘ž },𝜎(7)for π‘“π‘œπ‘Ÿ 𝑦 , πœ‡ , 𝜎 0 and π‘ž 0. This distribution function is characterized by alocation parameter πœ‡, a scale parameter 𝜎, and a shape parameter π‘ž, where π‘˜ is the normalizing constant and 𝑐is a constant which may depend on π‘ž. The Normal distribution is obtained from this distribution when π‘ž 2,whereas heavier (or lighter) tail distributions are produced for π‘ž 2 (π‘œπ‘Ÿ π‘ž 2). In particular, we obtain thedouble exponential distribution for q 1 and the uniform distribution for π‘ž .122

Mathematical Theory and ModelingISSN 2224-5804 (Paper)ISSN 2225-0522 (Online)Vol.8, No.8, 2018www.iiste.orgIn Equation (7), if we considerπ‘ž 2,1 Ξ²(8)(Elsalloukh, 2010), then the EPD is given as2𝑃(𝑦 πœ‡, 𝜎, 𝛽) π‘˜πœŽ 1𝑦 πœ‡ 1 𝛽𝑒π‘₯𝑝 { 𝑐 }.𝜎(9)This will be the basis for the study. The rationale for the expression for q is intended to enable us track thechanges in the characteristics of the distribution as a result of the values of 𝛽. Also, the constant c will be chosento ensure that it does not affect the variability or the scale parameter of the distribution. This would ensure theflexibility of the tails whether heavy or thinner tails. If 𝛽 0, the distribution becomes a Normal distribution; if𝛽 1, the distribution becomes a Laplace distribution, but if 𝛽 1, then the distribution turns to arectangular or uniform distribution.Figure 1: Exponential Distribution for some values of 𝛽Figure 1 presents a typical example of EPD for various values of the parameter, 𝛽. In Figure 1, it can beobserved that for a small value of 𝛽 (e.g., 𝛽 0.99), the EPD has a flat top, whiles for large values of 𝛽(e.g., 𝛽 1 and 𝛽 2) the EPD has a pencil-like top. Thus, for a decreasing values of 𝛽, the EPD approachesuniform or a rectangular distribution.Many researchers have adopted different forms of EPD, by adopting different values of the constant 𝑐 inEquation (9) which affect the scale parameter of the distribution as well as the normalizing constant π‘˜. For manyof the research, the choice of the constant 𝑐 depends on the shape parameter π‘ž. Vianelli (1963) developedEPDs with the constant 𝑐 deduced as 𝑐 1/π‘ž. Vianelli called the distribution β€œA Normal Distribution ofOrder π‘žβ€. Rahnamaei, Nematollahi and Farnoosh, (2012) adopted the distribution proposed by Vianelli in theirdata modelling. Giller (2005) adopted a case whereby 𝑐 1/2 which does not depend on π‘ž. Olosunde (2013)argued that there are limitation on using such family of EPDs, explaining that EPD exhibits thinner tails and careneeds to be taken to ensure that the tails are not affected by the choice of c. Due to Olosunde’s interest inanalysing data from heavy-tailed distribution, he adopted EPD with 𝑐 as a constant function, 𝑐(π‘ž), which heexplained to regulate the tail region of the distribution. This study will adopt the approach of Olosunde toestimate the constant 𝑐, but will ensure that it is estimated to make the variance of the EPD the same as the scaleparameter, 𝜎.Although, most research have addressed the characteristics of the EPDs of different kinds, information is rarelyprovided to address the reliability of the density function of their adopted distribution. Thus, the study willexamine the flexibility and the characteristics of EPD of various kinds and address the issue of its legitimacy andreliability. In the process, we will derive and estimate the parameters of the distribution with respect to a datasetand examine its fitness.123

Mathematical Theory and ModelingISSN 2224-5804 (Paper)ISSN 2225-0522 (Online)Vol.8, No.8, 2018www.iiste.org2. Methodology2.1 The Gamma DistributionThe gamma function is very important in mathematical statistics. It is a continuous extension to the factorialfunction, which is only defined for the non-negative integers. The gamma function (or gamma integral) is givenby Ξ“(s) y s 1 e y 𝑑𝑦 ,s 0,(10)0or sometimes 2Ξ“(𝑠) 2 𝑦 2𝑠 1 𝑒 𝑦 𝑑𝑦 ,𝑠 0.(11)0Also, the function Ξ“(s, t) 𝑦 s 1 e y 𝑑𝑦,(12)𝑑for all s 0and y 0 is the incomplete gamma function. Now, if s 1, then Ξ“(𝑠) (𝑠 1)Ξ“(𝑠 1). Forany non-negative integer, the logarithmic derivative of πœ“(𝑠) is the psi or digamma function denoted πœ“(𝑠)and given as𝑑Γ′ (𝑠)πœ“(𝑠) (ln(Ξ“(𝑠))) ,(13)𝑑𝑠Γ(𝑠)and expressed as 𝑒 𝑦𝑒 𝑠𝑦(14)πœ“(𝑠) ( ) 𝑑𝑦 .𝑦1 𝑒 𝑦0While there are other continuous extensions to the factorial function, the gamma function is the only one that isconvex for positive real numbers. Figure 2, presents a typical gamma function in the plane.Figure 2: Gamma function [Ξ“(𝑠)] in the whole complex planeFrom Figure 2, the function is defined for non-negative values of s, but undefined (discontinues) for somenegative values of s. The gamma function however plays a major role in the characteristics and properties ofEPD, as we will demonstrate in following Sections.2.2 The Legitimacy of the Exponential Power DistributionLet Y be a continuous random variable. The function 𝑓(𝑦) is said to be a proper or legitimate probabilitydensity function (pdf) of the continuous variable Y if 𝑓(𝑦) is positive for all values of y within ℝ𝑦 , that is𝑓(𝑦) 0 for 𝑦 ℝ (non-negativity) and if ℝ 𝑓(𝑦)𝑑𝑦 1.(15)𝑦124

Mathematical Theory and ModelingISSN 2224-5804 (Paper)ISSN 2225-0522 (Online)Vol.8, No.8, 2018www.iiste.orgThus, the EPD is a proper pdf of the continuous variable Y if 𝑃(𝑦 πœ‡, 𝜎, 𝛽) is for all values of y within ℝ𝑦 andif 2 𝑃(𝑦 πœ‡, 𝜎, 𝛽) 𝑑𝑦 π‘˜πœŽ 1 𝑦 πœ‡ 1 𝛽𝑒π‘₯𝑝 { 𝑐 } 𝑑𝑦 1.𝜎(16)Also, 𝑃(𝑦 πœ‡, 𝜎, 𝛽) 0 for all 𝑦 in the real line ℝ not in ℝ𝑦 . We note that probabilities are given by areasunder 𝑃(𝑦 πœ‡, 𝜎, 𝛽) as𝑏𝑃(π‘Ž π‘Œ 𝑏) 𝑃(𝑦 πœ‡, 𝜎, 𝛽)𝑑𝑦 .(17)π‘Žπ‘ŽWe also note the peculiarity that 𝑃(π‘Œ π‘Ž) π‘Ž 𝑃(𝑦 πœ‡, 𝜎, 𝛽)𝑑𝑦 0, for any arbitrary value π‘Ž. This can becircumvented by defining the probability on a small interval (π‘Ž 𝑦, π‘Ž 𝑦) around π‘Ž, where 𝑦 has a smallvalue. Thenπ‘Ž 𝑦𝑃(π‘Œ (π‘Ž 𝑦, π‘Ž 𝑦)) 𝑃(𝑦 πœ‡, 𝜎, 𝛽)𝑑𝑦 ,(18)π‘Ž 𝑦is properly defined.Given the two conditions for the legitimacy of the EPD, it can be deduce that if the integral 𝐼 𝑃(𝑦 πœ‡, 𝜎, 𝛽) 𝑑𝑦,(19) exists and is finite and strictly positive, then 1 𝐼 is called the normalizing constant. In this paper, the coefficientk in Equation (9) is the normalizing constant so that the area under the graph of the EPD is 1. This ensures thelegitimacy of the EPD.We will now derive an expression for the normalizing constant k.2.2 Deducing the Normalizing ConstantThe normalizing constant k, can be deduced by ensuring that 𝑦 πœ‡ π‘žπ‘˜πœŽ 1 𝑒π‘₯𝑝 { 𝑐 } 𝑑𝑦 1.𝜎 In relation to the absolute term 0π‘˜πœŽ 1 { 𝑒π‘₯𝑝 [ 𝑐 ( Letπ‘₯ 𝑐 (for which𝑦 πœ‡ π‘žπœŽπ‘‘π‘¦π‘‘π‘₯𝑦 πœ‡πœŽ1π‘ž , Equation (20) can be expressed as 𝑦 πœ‡ π‘žπ‘¦ πœ‡ π‘ž) ] 𝑑𝑦 𝑒π‘₯𝑝 [ 𝑐 () ] 𝑑𝑦} 1.𝜎𝜎0) so that 𝑐 1 π‘₯ (𝑦 πœ‡ π‘žπœŽ1 1 π‘žπ‘₯ π‘ž) and 𝑐 𝑦 πœ‡πœŽ(21). Thus, we can deduce that 𝑦 πœŽπ‘ 11 π‘ž π‘₯ π‘ž 1 . πœŽπ‘ (20)Thus, we will have Equation (21) as0 111111 1 1π‘˜πœŽ 1 [ 𝑒 π‘₯ πœŽπ‘ π‘ž π‘₯ π‘ž 𝑑π‘₯ 𝑒 π‘₯ πœŽπ‘ π‘ž π‘₯ π‘ž 𝑑π‘₯ ] 1,π‘žπ‘ž 00 1111 1 1π‘˜πœŽ 1 πœŽπ‘ π‘ž [ π‘₯ π‘ž 𝑒 π‘₯ 𝑑π‘₯ π‘₯ π‘ž 𝑒 π‘₯ 𝑑π‘₯ ]π‘ž 0 1,0 111 1 1 1π‘˜ 𝑐 π‘ž [ π‘₯ π‘ž 𝑒 π‘₯ 𝑑π‘₯ π‘₯ π‘ž 𝑒 π‘₯ 𝑑π‘₯ ]π‘ž 0 1.(22)Since the two integrals in Equation (22) are symmetrical, we should have 11 1 12π‘˜ 𝑐 π‘ž [ π‘₯ π‘ž 𝑒 π‘₯ 𝑑π‘₯ ] 1.π‘ž0(23)From Equation (10), the integral in Equation (23) are a family of gamma function given as1251 1 π‘žπ‘₯ π‘ž πœ‡,

Mathematical Theory and ModelingISSN 2224-5804 (Paper)ISSN 2225-0522 (Online)Vol.8, No.8, 2018www.iiste.org 11 1 π‘₯ π‘ž 𝑒 π‘₯ 𝑑π‘₯ Ξ“ ( ).π‘ž0Thus,1 π‘ž2π‘˜π‘ 1 1[ Ξ“ ( )] 1.π‘ž π‘ž(24)Thus, the normalizing constant k is given asπ‘˜ 112𝑐 π‘žNow, for π‘ž π‘˜ 21 𝛽1[Ξ“ ( 1)]π‘ž.(25)so, we have1(1 𝛽)2𝑐 23 𝛽[Ξ“ ()]2.(26)From Equations (25) and (26), the normalizing constant, k, is undefined for some values q and 𝛽, respectively.This makes the EPD illegitimate. Figure 3 presents the graphical relationship between k and the shape parameter,𝛽. It can be observed that k is undefined for some negative values of 𝛽, and for higher values of 𝛽 (𝛽 80).This makes the integral over ℝ not equal to 1, and thus, the EPD is illegitimate for such values of 𝛽.Figure 3: Relationship between the normalized constant, k and the shape parameter, 𝛽2.3 The Central Moment of the Exponential Power DistributionThe ith central moment of a random variable Y for EPD function, 𝑃(𝑦 πœ‡, 𝜎, π‘ž) is given by 𝐸[(𝑦 πœ‡)𝑖 ] (𝑦 πœ‡)𝑖 𝑃(𝑦 πœ‡, 𝜎, π‘ž) 𝑑𝑦, π‘˜πœŽ 1 (𝑦 πœ‡)𝑖 𝑒π‘₯𝑝 { 𝑐 𝑦 πœ‡ π‘ž } 𝑑𝑦.𝜎Since the integral is symmetrical about the location parameter, πœ‡, we have 𝑦 πœ‡ π‘žπΈ[(𝑦 πœ‡)𝑖 ] [1 ( 1)𝑖 ]π‘˜πœŽ 1 (𝑦 πœ‡)𝑖 𝑒π‘₯𝑝 { 𝑐 () } 𝑑𝑦,πœŽπœ‡so that 𝐸[(𝑦 πœ‡)𝑖 ] 0 for odd values of i.126(27)

Mathematical Theory and ModelingISSN 2224-5804 (Paper)ISSN 2225-0522 (Online)Vol.8, No.8, 2018By standardization, setting z (𝑦 πœ‡πœŽwww.iiste.org) so that 𝑧 𝑖 (𝑦 πœ‡)π‘–πœŽπ‘–and 𝜎 𝑖 𝑧 𝑖 (𝑦 πœ‡)𝑖 . Now, πœŽπ‘‘π‘§ 𝑑𝑦 and 𝐸(𝑧) 0.Substituting these deductions into Equation (27), we have 𝐸[(𝑦 πœ‡)𝑖 ] π‘˜πœŽ 1 [1 ( 1)𝑖 ] 𝑧 𝑖 𝜎 𝑖 𝑒π‘₯𝑝{ 𝑐𝑧 π‘ž } πœŽπ‘‘π‘§,0 π‘˜πœŽ 𝑖 [1 ( 1)𝑖 ] 𝑧 𝑖 𝑒π‘₯𝑝{ 𝑐𝑧 π‘ž } 𝑑𝑧,(28)0Now, we let π‘₯ 𝑐𝑧 π‘ž , so that 𝑐 1 π‘₯ 𝑧 π‘ž . Thus, 𝑧 𝑐 1 1 π‘žπ‘₯ π‘ž ,𝑖𝑖𝑧 𝑖 𝑐 π‘ž π‘₯ π‘ž and𝑑𝑧𝑑π‘₯1π‘ž11 1π‘žπ‘₯π‘ž . 𝑐 Equation (28) then gives𝑖1 π‘žπΈ[(𝑦 πœ‡)𝑖 ] π‘˜πœŽ 𝑖 [1 ( 1)𝑖 ]𝑐 π‘ž 𝑐 1 𝑖 1 1 π‘₯ π‘₯ π‘ž π‘₯ π‘ž 𝑒 𝑑π‘₯,π‘ž 0𝑖 1 𝑖 11 𝑖 1π‘˜πœŽ [1 ( 1)𝑖 ]𝑐 π‘ž π‘₯ π‘ž 𝑒 π‘₯ 𝑑π‘₯.π‘ž0(29)Equation (29) simplifies as𝐸[(𝑦 πœ‡)𝑖 ] 𝑖 11 𝑖𝑖 1 π‘˜πœŽ [1 ( 1)𝑖 ]𝑐 π‘ž [Ξ“ ()] .π‘žπ‘žMaking substitution for k, we obtain𝐸[(𝑦 πœ‡)𝑖 ] 12𝑐 π‘žπ‘– 1 11𝑖 1 𝜎 𝑖 [1 ( 1)𝑖 ]𝑐 π‘ž [ Ξ“ ()],1 1π‘žπ‘ž[ Ξ“ ( )]π‘ž π‘žwhich simplifies as𝑖 1)] 𝜎 𝑖 [1 ( 1)𝑖 ]π‘ž( 1 ).12𝑐 π‘ž[Ξ“ ( )]π‘ž[Ξ“ (𝑖]𝐸[(𝑦 πœ‡) For q 21 𝛽(30)so, we have(𝑖 1)(1 𝛽)[Ξ“ ()] 𝜎 𝑖 [1 ( 1)𝑖 ]2𝑖]𝐸[(𝑦 πœ‡) ( 1 𝛽 ).1 𝛽22[Ξ“ ()]𝑐2(31)We will now use the ith central moment to derive the mean, variance, skewness and kurtosis of the EPD. Thefollowing sections present the results.2.3.1 Mean and Variance of the Exponential Power DistributionIt can be deduced from Equation (30) that 𝐸(𝑦) πœ‡ when 𝑖 1. In Equation (30) again, if 𝑖 2, then2 1[Ξ“ ()] 𝜎 2 [1 ( 1)2 ]π‘ž2]𝐸[(𝑦 πœ‡) ( 1 ).12𝑐 π‘ž[Ξ“ ( )]π‘žThus,3[Ξ“ ( )] 𝜎 2π‘žπ‘£π‘Žπ‘Ÿ(𝑦) ( 1 ) ,(32)1 [Ξ“ ( )] 𝑐 π‘žπ‘ž127

Mathematical Theory and ModelingISSN 2224-5804 (Paper)ISSN 2225-0522 (Online)Vol.8, No.8, 2018𝑐 21 𝛽3[Ξ“ ( )]π‘žπœŽ2 .1[Ξ“ ( )]π‘ž2 π‘ž Now, for π‘ž www.iiste.org(33)so, we have𝑐 1 𝛽2π‘£π‘Žπ‘Ÿ(𝑦) 3(1 𝛽)[Ξ“ ()]2𝜎 2.1 𝛽[Ξ“ ()]2(34)From Equation (33), it can be observed that, the π‘£π‘Žπ‘Ÿ(𝑦) is affected by the shape parameter, q, and the constant,c. We want to derive c such that π‘£π‘Žπ‘Ÿ(𝑦) 𝜎 2 .Let3[Ξ“ ( )]π‘žβ„Ž .(35)1[Ξ“ ( )]π‘žThus, if β„Ž 1, then π‘£π‘Žπ‘Ÿ(𝑦) 𝜎 2 . Therefore, we would find c such that23𝑐 π‘ž [Ξ“ ( )]π‘ž 1.(36)1[Ξ“ ( )]π‘žThis gives2 π‘žπ‘ π‘ž 23𝑐 [Ξ“ ( )]π‘žπ‘ž 21 [Ξ“ ( )]π‘ž,(37)or113(1 𝛽) 1 𝛽1 𝛽 1 𝛽𝑐 [Ξ“ ()][Ξ“ ()].22(38)2.3.2 Skewness and Kurtosis of the Exponential Power DistributionThe third central moment given byπœ‡3 𝐸[(𝑦 πœ‡)3 ],(39)is used to determine the symmetry of the distribution. As we know, πœ‡3 alone is a poor measure of skewnesssince the size is influenced by the units used to measure the values of X. To make this measure dimensionless, weuseπ‘Ž3 𝐸[(𝑦 πœ‡)3 ] π‘‰π‘Žπ‘Ÿ(𝑦)3,(40)which is a measure of lack of symmetry. Since 𝐸[(𝑦 πœ‡)3 ] 0, π‘Ž3 0.Generally, the coefficient of kurtosis, also known as the fourth standardized comulant, is given byπ‘Ž4 𝐸[(𝑦 πœ‡)4 ] 3.π‘£π‘Žπ‘Ÿ(𝑦)2(41)In terms of EPD, the coefficient of kurtosis is given by (𝑦 πœ‡)4 𝑃(𝑦 πœ‡, 𝜎, π‘ž) π‘‘π‘¦π‘Ž4 3.π‘£π‘Žπ‘Ÿ(𝑦)2(42)This measures the nature of the spread of the values around the mean. Thus, it is a measure of the peakedness ofEPD or how heavy the tails of EPD are. If a random population has kurtosis above or below zero (0), it cannot be128

Mathematical Theory and ModelingISSN 2224-5804 (Paper)ISSN 2225-0522 (Online)Vol.8, No.8, 2018www.iiste.orgadequately represented by a normal distribution. From Equation (30),5[Ξ“ ( )] 𝜎 4π‘ž4]𝐸[(𝑦 πœ‡) ( 1 ) .(43)1 [Ξ“ ( )] 𝑐 π‘žπ‘žThus, the coefficient of Kurtosis could then be deduced as5[Ξ“ ( )] 𝜎 4π‘ž1 ( 1 )[Ξ“ ( )] 𝑐 π‘žπ‘žπ‘Ž4 3.(44)π‘£π‘Žπ‘Ÿ(𝑦)2Substituting the expression for var(y) into Equation (44), we have5[Ξ“ ( )] 𝜎 4π‘ž1 ( 1 )[Ξ“ ( )] 𝑐 π‘žπ‘žπ‘Ž4 2 3,3[Ξ“ ( )]2π‘žπœŽ[1 (𝑐 1 π‘ž ) ][Ξ“ ( )]π‘žwhich simplifies asπ‘Ž4 51[Ξ“ ( )] [Ξ“ ( )]π‘žπ‘ž3[[Ξ“ ( )]]π‘ž2 3.(45)In terms of 𝛽, we haveπ‘Ž4 5(1 𝛽)1 𝛽[Ξ“ ()] [Ξ“ ()]223(1 𝛽)[[Ξ“ ()]]22 3.(46)Table 1: Estimation of the values of the constant, c, the normalized constant, k and the kurtosis for some valuesof 𝛽𝛽qckKurtosis (π‘Ž4 )-1.0 .257129

Mathematical Theory and ModelingISSN 2224-5804 (Paper)ISSN 2225-0522 (Online)Vol.8, No.8, 2018www.iiste.orgTable 1 presents some estimations of the normalizing constant, k, the constant c which ensures that π‘£π‘Žπ‘Ÿ(𝑦) 𝜎 2 and the coefficient of kurtosis of the EPD for some values of 𝛽. The estimations were based on thederivations of π‘ž, π‘˜, 𝑐 and π‘Ž4 in Equations (8), (26), (38) and (46), respectively. The values were estimatedbased on the values of 𝛽 of -1, -0.8, -0.6, , 2. From the table, it can be observed that for 𝛽 0, thecoefficient of kurtosis, 4, is zero (0), indicating a mesokurtic distribution with identical distribution as that of theNormal. Also, for negative values of 𝛽, π‘Ž4 is also negative, with exception of 𝛽 1 for which thedistribution is rectangular and hence π‘Ž4 is undefined. For positive values of 𝛽, π‘Ž4 is also positive. For 𝛽 1,π‘Ž4 is equal to 3, indicating a do

1. Department of Statistics, University of Cape Coast, Cape Coast, Ghana 2. Directorate of Academic Planning and Quality Assurance, Cape Coast Technical University, P. O. Box AD50, Cape Coast, Ghana * E-mail of the corresponding author: mykemanford@yahoo.com Abstract

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