A NEW CLASS OF SEMI-PARAMETRIC ESTIMATORS OF THE SECOND ORDER PARAMETER *

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1 PORTUGALIAE MATHEMATICA Vol. 60 Fasc Nova Série A NEW CLASS OF SEMI-PARAMETRIC ESTIMATORS OF THE SECOND ORDER PARAMETER * M.I. Fraga Alves M. Ivette Gomes ad Laures de Haa Abstract: The mai goal of this paper is to develop uder a semi-parametric cotext asymptotically ormal estimators of the secod order parameter a parameter related to the rate of covergece of maximum values liearly ormalized towards its limit. Asymptotic ormality of such estimators is achieved uder a third order coditio o the tail 1 F of the uderlyig model F ad for suitably large itermediate raks. The class of estimators itroduced is depedet o some cotrol or tuig parameters ad has the advatage of providig estimators with stable sample paths as fuctios of the umber k of top order statistics to be cosidered for large values of k; such a behaviour makes obviously less importat the choice of a optimal k. The practical validatio of asymptotic results for small fiite samples is doe by meas of simulatio techiques i Fréchet ad Burr models. 1 Itroductio I Statistical Extreme Value Theory we are essetially iterested i the estimatio of parameters of rare evets like high quatiles ad retur periods of high levels. Those parameters deped o the tail idex γ = γf of the uderlyig model F. which is the shape parameter i the Extreme Value EV distributio fuctio d.f. { exp 1 + γ x 1/γ} 1 + γ x > 0 if γ Gx G γ x := exp exp x x R if γ = 0. Received: Jue ; Revised: February AMS Subject Classificatio: Primary 60G70 62G32; Secodary 62G05 62E20 65C05. Keywords ad Phrases: extreme value theory; tail iferece; semi-parametric estimatio; asymptotic properties. * Research partially supported by FCT/POCTI/FEDER.

2 194 M.I. FRAGA ALVES M. IVETTE GOMES ad LAURENS DE HAAN This d.f. appears as the o-degeerate limitig d.f. of the sequece of maximum values {X : = maxx 1 X 2... X } 1 liearly ormalized with {X i } i 1 a sequece of idepedet idetically distributed i.i.d. or possibly weakly depedet radom variables r.v. s Galambos [9]; Leadbetter ad Nadagopala [21]. Wheever there is such a o-degeerate limit we say that F is i the domai of attractio of G γ ad write F DG γ. Puttig Ut := F 1 1/t for t > 1 where F t = if{x: F x t} deotes the geeralized iverse fuctio of F we have for heavy tails i.e. for γ > F DG γ iff 1 F RV 1/γ iff U RV γ where RV β stads for the class of regularly varyig fuctios at ifiity with idex of regular variatio equal to β i.e. fuctios g. with ifiite right edpoit ad such that lim t gt x/gt = x β for all x > 0. The coditios i 1.2 characterize completely the first order behaviour of F Gedeko [10]; de Haa [17]. The secod order theory has bee worked out i full geerality by de Haa ad Stadtmüller [18]. Ideed for a large class of heavy tail models there exists a fuctio At 0 of costat sig for large values of t such that 1.3 l Utx l Ut γ l x lim t At = x 1 for every x > 0 where 0 is a secod order parameter which also eeds to be properly estimated from the origial sample. The limit fuctio i 1.3 must be of the stated form ad At RV Geluk ad de Haa [11]. Notice that as icreases the rate of covergece i the first order approximatio icreases as well ad this is importat for approximatios i real problems. Here for part of our results we shall assume the validity of a third order framework i.e. we shall assume there is a fuctio Bt 0 also of costat sig for large values of t ad 1.4 lim t l Ut x l Ut γ l x At Bt x 1 = 1 β { x +β } 1 + β x 1. The Bt RV β β 0. Uder the validity of 1.3 ad 1.4 we have for every x > 0 ad as t l Ut x l Ut = γ l x + At x 1 + At Bt Hx; β 1 + o1

3 SEMI-PARAMETRIC ESTIMATORS 195 where Hx; β = 1 β { x +β } 1 + β x 1. For heavy tails estimatio of the tail idex γ may be based o the statistics 1.5 M k := 1 k k [ ] l X i+1: l X k: R + i=1 where X i: 1 i is the sample of ascedig order statistics o.s. associated to our origial sample X 1 X 2... X. These statistics were itroduced ad studied uder a secod order framework by Dekkers et al. [5]. For more details o these statistics ad the way they may be used to build alteratives to the Hill estimator give by 1.5 ad = 1 Hill [20] see Gomes ad Martis [12]. I this paper we are iterested i the estimatio of the secod order parameter i 1.3. The secod order parameter is of primordial importace i the adaptive choice of the best threshold to be cosidered i the estimatio of the tail idex γ like may be see i the papers by Hall ad Welsh [19] Beirlat et al. [1] [2] Drees ad Kaufma [7] Daielsso et al. [4] Draisma et al. [6] Guillou ad Hall [16] amog others. Also most of the recet research devised to improve the classical estimators of the tail idex try to reduce the mai compoet of their asymptotic bias which also depeds strogly o. So a a priori estimatio of is eeded or at least desirable for the adequate reductio of bias. Some of the papers i this area are the oes by Beirlat et al. [3] Feuerverger ad Hall [8] Gomes ad Martis [12] [13] ad Gomes et al. [15]. All over the paper ad i order to simplify the proof of theoretical results we shall oly assume the situatio β < 0. We shall also assume everywhere that k is a itermediate rak i.e. 1.6 k = k k/ 0 as. The startig poit to obtai the class of estimators we are goig to cosider is a well-kow expasio of M k for ay real > 0 valid for itermediate k M k = γ + γ σ 1 1 P + γ 1 µ 2 AY k: + o p A/k k where P is asymptotically a stadard ormal r.v. cf. sectio 2 below where the otatio is explaied. The reasoig is the similar to the oe i Gomes et al. [14]: first for sequeces k = k with k A/k = O1 as

4 196 M.I. FRAGA ALVES M. IVETTE GOMES ad LAURENS DE HAAN this gives a asymptotically ormal estimator of a simple fuctio of γ; but by takig sequeces k of greater order tha the previous oes i.e. such that 1.7 k A/k as we ca emphasize other parts of this equatio as follows. First we get rid of the first term o the right by composig a liear combiatio of powers of two differet M k s i.e. for two differet suitably ormalized. We have here cosidered for positive real τ ad θ 1 1 M k τ AY k: M τ/θ1 k τ γ τ 1 µ 2 µ2 which is a fuctio of both parameters of the model γ ad. We the get rid of the ukow AY k: ad of γ by composig for positive real values θ 1 θ 2 both differet from 1 a quotiet of the type 1.8 T θ 1θ 2 τ k := M k M k τ τ/θ1 M τ/θ1 k M τ/θ2 k which uder coditios 1.6 ad 1.7 coverges i probability towards t θ1 θ 2 := µ 2 µ 2 µ µ2 2 ad where the admissible values of the tuig parameters are θ 1 θ 2 τ R + θ 1 1 ad θ 1 θ 2. We thus obtai a cosistet estimator of a fuctio of which leads to a cosistet estimator of as developed i sectio 2. I sectio 3 a somewhat more refied aalysis agai o the lies of Gomes et al. [14] usig third order regular variatio brigs the terms γ σ 1 1 k P back ito play ad this will lead to a proof of the asymptotic ormality of our estimators. We shall pay particular attetio to the statistic obtaied for = 1 θ 1 = 2 ad θ 2 = 3 which ivolves oly the first three momet statistics M i k i = also hadled i Draisma et al. [6] uder a differet geeral framework ad for the estimatio of

5 SEMI-PARAMETRIC ESTIMATORS 197 γ R. We shall also advace with some idicatio o a possible way to choose the cotrol parameters i order to get estimators with stable sample paths ad flat Mea Square Error MSE patters for large values of k the umber of top order statistics used i their costructio. Fially the practical validatio of asymptotic results for small fiite samples is doe i sectio 4 by meas of simulatio techiques i Fréchet ad Burr models. 2 A class of semi-parametric estimators of the secod order parameter Let W deote a expoetial r.v. with d.f. F W x = 1 exp x x > 0 ad with the same otatio as i Gomes et al. [14] let us put := E[W ] = Γ + 1 σ 1 := µ 2 := E σ 2 := Var[W ] = [ W 1 e W 1 [ Var Γ2 + 1 Γ W 1 e W 1 ] = Γ ] = µ 3 2 µ 2 2 with 2.5 ad 2.6 µ 3 := E = [ 1 W 2 e W l { Γ ] } [ ] 1 e µ 4 β := E β W 1 +βw 1 ew 1 + β = 1 µ 2 + β µ 2. β if = 1 if 1 The uder the third order coditio i 1.4 assumig that 1.6 holds ad usig the same argumets as i Dekkers et al. [5] i lemma 2 of Draisma

6 198 M.I. FRAGA ALVES M. IVETTE GOMES ad LAURENS DE HAAN et al. [6] ad more recetly i Gomes et al. [14] we may write the distributioal represetatio θ M k θ γθ 2.7 τ/θ = 1 + τ + τ θ σ1 θ γ µ2 θ 1 P θ k + τ 2γ 2 A2 /k A/k + τ γ σ2 θ A/k k P θ θ 1 µ 3 θ + τ θ µ2 θ 2 1+o p 1 + τ γ µ4 θ β A/k B/k 1+o p1 where P θ ad P θ are asymptotically stadard Normal r.v. s ad = µj µ j σ 1 = σ1 σ 2 j = 2 3 µ 4 β = µ4 β = σ2 If we ow take the differece betwee two such expressios we get a r.v. covergig towards 0: D θ 1θ 2 τ k := 2.8 M γ τ/θ1 k = τ 1 σ P k θ 1 + τ γ + τ γ + τ 2 γ 2 M. k γ σ1 P θ 2 µ 2 µ 2 τ/θ2 A/k σ 2 P σ 2 P θ 2 A/k k 1 µ 3 + τ θ 1 µ µ 3 τ θ 2 µ 2 2 A 2 /k 1+o p 1 + τ µ 4 θ γ 1 β µ 4 β A/k B/k 1+o p 1.

7 SEMI-PARAMETRIC ESTIMATORS 199 If we assume that 1.7 holds the secod term i the right had side of 2.8 is the domiat oe ad D θ 1θ 2 τ k A/k 2.9 = τ γ µ 2 µ 2 1 τ σ θ + 1 P σ1 P k A/k θ 1 θ 2 + τ 2 γ 2 1 µ 3 + τ θ 1 µ µ 3 τ θ 2 µ 2 2 A/k 1+o p 1 + τ γ µ 4 β µ 4 β B/k 1+o p 1. Cosequetly for θ 1 θ 2 the statistic i 1.8 which may be writte as 2.10 T θ 1θ 2 τ k = D1θ 1τ k D θ 1θ 2 τ k coverges i probability as towards 2.11 t θ1 θ 2 := µ2 µ 2 µ 2 µ 2 = d 1θ1 d θ1 θ 2 idepedetly of τ where 2.12 d θ1 θ 2 := µ 2 µ 2. Straightforward computatios lead us to the expressio 2.13 t θ1 θ 2 = θ 2 θ θ 1 1 θ θ 2 θ 1 θ 2 θ 1 1 θ2 1 θ 2 θ 1 + θ 1. We have 2.14 lim t 0 θ 1 θ 2 = θ 1 1 ; θ 2 θ 1 lim t θ 1 θ 2 = θ 2θ 1 1 θ 2 θ 1 ad for egative values of ad >0 t ;θ1 θ 2 is always a decreasig icreasig fuctio of provided that 1<θ 1 <θ 2 θ 1 >θ 2 >1.

8 200 M.I. FRAGA ALVES M. IVETTE GOMES ad LAURENS DE HAAN We have thus got a cosistet estimator of a fuctio of which eeds to be iverted i.e. the estimator of to be studied i the followig sectio is 2.15 θ1θ 2τ T k := t θ 1 θ 2 T θ 1θ 2 τ k provided that θ 1 1 T θ 2 θ 1 θ 1 θ 2 τ k < θ 2θ 1 1 θ 2 θ 1 i order to get the right sig for the -estimator. The easiest situatio is the oe associated to values θ 1 θ 2 such that θ 2 θ 1 = 1 ad θ 2 1 = 2 look at expressio 2.11 i.e. to the values θ 1 = 2 θ 2 = 3 for which we get 2.16 t = t 23 = = which for ay > 0 must be betwee 1 ad 3 to provide by iversio egative values of. We the get a explicit aalytic expressio for the estimator of. More specifically we get τ T k := 1 23τ 2 T k 3 T 23τ k 1/ provided that 1 T 23τ k < 3. For the particular case = 1 we have 123τ T k := 3 T 123τ k T 123τ k 3 provided that 1 T 123τ k < 3. We have thus proved the followig Theorem 2.1. Suppose that the secod order coditio 1.3 holds with < 0. For sequeces of itegers k = k satisfyig k = o ad k A/k as we have lim θ 1θ 2 τ T k = i probability for ay τ > 0 R + ad θ 1 θ 2 R + \{1} θ 1 θ 2 with θ 1θ 2 τ T k defied i 2.15 ad with a explicit aalytic expressio give by 2.17 for θ 1 θ 2 = 2 3.

9 SEMI-PARAMETRIC ESTIMATORS The asymptotic ormality of the estimators of the secod order parameter 3.1 From 2.9 ad uder the validity of 1.7 D θ 1θ 2 τ k τ γ 1 A/k = d θ 1 θ 2 + where + γ k A/k W θ 1θ 2 { u θ1 θ 2 τ A/k + v θ1 θ 2 β B/k } 1+o p ad 3.4 W θ 1θ 2 u θ1 θ 2 τ := 1 2 γ := σ1 P θ 1 σ1 θ 2 P { 1 µ 3 + τ θ 1 µ µ 3 τ θ 2 µ 2 2 } v θ1 θ 2 β := µ 4 β µ 4 β. The sice T θ 1θ 2 τ k = D 1θ 1τ k/d θ 1θ 2 τ k we have wheever 3.5 k = k k/ 0 k A/k as T θ 1θ 2 τ k = t θ1 θ 2 γ + k A/k 3.6 { } W 1θ 1 t θ1 θ 2 W θ 1θ 2 1 d θ1 θ 2 { u1θ1 τ t θ1 θ + 2 u θ1 θ 2 τ A/k d θ1 θ 2 + v } 1θ 1 β t θ1 θ 2 v θ1 θ 2 β B/k d θ1 θ 2 1+o p 1. From the asymptotic covariace betwee σ 1 P ad σ 1 P Gomes ad Martis [12] give by θ 1 + θ 2 Γ θ 1 + θ θ 1 θ 2 Γ Γ see

10 202 M.I. FRAGA ALVES M. IVETTE GOMES ad LAURENS DE HAAN we easily derive the asymptotic covariace betwee W 1θ 1 ad W θ 1θ 2 give by σ W 1θ1 θ 2 = 1 θ 1+1 Γ θ 1 +1 θ1 2 Γ Γθ θ 2+1 Γ θ θ2 2 Γ Γθ 2 Γ2 2 θ1 3 Γ2 + θ 1+θ 2 Γ θ 1 +θ 2 θ1 2 θ2 2 Γθ Γ θ1 θ1 θ2 The asymptotic variace of W θ 1θ 2 is σw 2 θ 1 θ 2 = 2 Γ2 θ1 3 Γ2 + Γ2 θ2 3 Γ2 θ 1+θ 2 Γ θ 1 +θ θ1 2 θ2 2 Γθ 1 Γ θ1 θ 2 Cosequetly if apart from the previous coditios i 3.5 we also have 3.9 lim k A 2 /k = 0 ad lim k A/k B/k = 0 there is a ull asymptotic bias ad k A/k T θ 1θ 2 τ d 3.10 k t θ1 θ 2 Z where Z is a Normal r.v. with ull mea ad variace give by σ σ 2 = γ2 W 2 + t 2 1θ1 θ 1 θ 2 σw 2 2 t θ1 θ2 θ1 θ 2 σ W 1θ1 θ T θ 1 θ 2 2 d 2 θ 1 θ 2 with t θ1 θ 2 d θ1 θ 2 σ W 1θ1 θ 2 ad σw 2 θ 1 θ 2 give i ad 3.8 respectively. I the more geeral case 3.12 lim k A 2 /k = λ 1 ad lim k A/k B/k = λ2 we have to take ito accout a o-ull asymptotic bias i.e. { } k A/k T θ 1θ 2 τ k t θ1 θ 2 is asymptotically Normal with mea value equal to 3.13 µ T θ1 θ 2 τ = λ 1 u T θ1 θ 2 τ + λ 2 v T θ1 θ 2

11 SEMI-PARAMETRIC ESTIMATORS 203 where u T θ1 θ 2 τ u T θ 1 θ 2 τ = u 1θ 1 τ t θ1 θ 2 u θ1 θ 2 τ d θ1 θ 2 v T θ1 θ 2 v T θ1 θ 2 β = v 1θ 1 β t θ1 θ 2 v θ1 θ 2 β d θ1 θ 2 ad variace give by 3.11 as well as before. Figure 1 illustrates for θ 1 = 2 ad θ 2 = 3 the behaviour of σ T θ1 θ 2 /γ γ u T θ1 θ 2 τ ad v T θ 1 θ 2 β as fuctios of for τ = = β = 1. Fig. 1 : σ T θ1 θ 2 /γ γu T θ1 θ 2 τ ad v T θ 1 θ 2 β as fuctios of for θ 1 = 2 θ 2 = 3 τ = 1 ad assumig = β = 1. The it follows that for the -estimator θ 1θ 2 τ T k defied i 2.15 we have that uder 3.12 k A/k { θ } 1θ 2 τ T k is asymptotically Normal with mea value equal to 3.16 µ θ 1θ 2 τ T = µ T θ1 θ 2 τ /t θ 1 θ 2 =: λ 1 u θ 1θ 2 τ T + λ 2 v θ 1θ 2 T ad with variace give by σ Tθ 2 σt θ1 1 θ 2 = θ 2 t θ 1 θ 2

12 204 M.I. FRAGA ALVES M. IVETTE GOMES ad LAURENS DE HAAN where t θ 1 θ 2 is such that t 2 θ 1 θ 2 1 θ 2 θ 1 1 θ 2 1 θ 2 θ 1 + θ 1 = { = θ 1 θ 2 θ 1 θ θ θ 2 θ 1 +1 θ 2 θ 1 1 θ 2 θ θ 2+θ 1 1 θ 2 θ θ 2 θ 1 1 }. For the particular but iterestig case = 1 θ 1 = 2 θ 2 = 3 ad uder the same coditios as before we have that with 123τ T k give i 2.18 k A/k { 123τ } T k is asymptotically Normal with variace give by 3.18 σ 2 T123 = γ The asymptotic bias is either ull or give by 3.16 accordig as 3.9 or 3.12 hold respectively. I Figure 2 we preset asymptotic characteristics of θ 1θ 2 τ T k for the same particular values of the cotrol parameters amely θ 1 = 2 θ 2 = 3 ad τ = = β = 1. Fig. 2 : σ T θ1 θ 2 /γ γu θ 1θ 2 τ T ad v θ 1θ 2 T β as fuctios of for θ 1 = 2 θ 2 = 3 τ = 1 ad assumig = β = 1.

13 SEMI-PARAMETRIC ESTIMATORS 205 We have thus proved the followig Theorem 3.1. Suppose that third order coditio 1.4 holds with < 0. For itermediate sequeces of itegers k = k satisfyig 3.19 lim k A/k = lim k A 2 /k = λ 1 fiite lim k A/kB/k = λ2 fiite we have that for every positive real umbers θ 1 θ 2 both differet from 1 ad τ > 0 { k A/k θ } 1θ 2 τ 3.20 T k is asymptotically ormal with mea give i 3.16 ad with variace give i Note that the variace does ot deped o λ 1 or λ 2. Remarks: 1. We agai ehace the fact that for ay τ > 0 the statistic T θ 1θ 2 τ k i 1.8 coverges i probability to the same limit. This leads to a adequate cotrol maagemet about the parameter τ which ca be useful i the study of the exact distributioal patters of this class of estimators. 2. If we let τ 0 we get the statistic 3.21 M l k l T θ 1θ 2 0 k := M l 1 / k θ 1 l M / k θ 1 M k / θ 2 ad Theorems 2.1 ad 3.1 hold true with τ replaced by 0 everywhere. 4 A illustratio of distributioal ad sample path properties of the estimators We shall preset i Figures 3 ad 4 the simulated mea values ad root mea square errors of the estimators 123τ T k i 2.18 τ = for a sample of size = 5000 from a Fréchet model F x = exp x 1/γ x 0 with γ = 1 = 1 ad a Burr model F x = x /γ 1/ x 0 also with = 1 ad γ = 1 respectively. Simulatios have bee carried out with 5000 rus.

14 206 M.I. FRAGA ALVES M. IVETTE GOMES ad LAURENS DE HAAN Notice that we cosider i both pictures values of k For smaller values of k we get high volatility of the estimators characteristics ad admissibility probabilities associated to 2.18 slightly smaller tha oe. Those probabilities are equal to oe wheever k 4216 i the Fréchet model ad k 2986 i the Burr model. Fig. 3 : Simulated mea values left ad mea square errors right of 123τ T k i 2.18 for τ = ad for a sample of size = 5000 from a Fréchet1 model = 1. Fig. 4 : Simulated mea values left ad mea square errors right of 123τ T k i 2.18 for τ = ad for a sample of size = 5000 from a Burr model with = 1 ad γ = 1. I Figure 5 we picture for k 2000 the simulated mea values ad root mea square errors of the same estimators for the same sample size but for a Burr

15 SEMI-PARAMETRIC ESTIMATORS 207 model with γ = 1 ad = 0.5. Here we get admissibility probabilities equal to oe for k Fig. 5 : Simulated mea values left ad mea square errors right of 123τ T k i 2.18 for τ = ad for a sample of size = 5000 from a Burr model with = 0.5 ad γ = 1. The previous pictures seem to suggest the choice τ = 0 for the tuig parameter τ. Notice however that such a choice is ot always the best oe as may be see from Figure 6 which is equivalet to Figure 5 but for a Burr model with γ = 1 ad = 2. This graph is represeted for k 4000 sice the admissibility probabilities of the estimators uder play are all equal to oe provided that k However sice values of with such magitude are ot commo i practice the choice τ = 0 seems to be a sesible oe. Fig. 6 : Simulated mea values left ad mea square errors right of 123τ T k i 2.18 for τ = ad for a sample of size = 5000 from a Burr model with = 2 ad γ = 1.

16 208 M.I. FRAGA ALVES M. IVETTE GOMES ad LAURENS DE HAAN We ayway advise the plot of sample paths of 123τ T k for a few values of τ like for istace the oes metioed before τ = ad 2 ad the choice of the value of τ which provides the highest stability i the regio of large k values for which we get admissible estimates of. Fially i Figure 7 we picture for the values of k which provide admissible estimates of a sample path of the same estimators for the same sample size = 5000 ad for two geerated samples oe from a Fréchet model with γ = 1 = 1 ad aother from a Burr model with = 0.5 ad γ = 1. Fig. 7 : Sample path of the estimators 123τ T k i 2.18 τ = for oe sample of size = 5000 from a Fréchet model with γ = 1 = 1 left ad aother sample from a Burr model with = 0.5 ad γ = 1 right. We have also carried out a large scale simulatio based o a multi-sample simulatio of size replicates with 5000 rus each for the estimators associated to the cotrol parameters θ 1 θ 2 = which provide the explicit expressio i 2.18 ad for τ = ad 6. The estimators 123τ T respectively. k τ = ad 6 will be here deoted by j k 1 j 5 I Tables 1 ad 2 we preset for a Fréchet paret with γ = 1 for which = 1 the simulated distributioal properties of the five estimators computed at the optimal level i.e. of j 0 := j k j j 0 k 0 := arg mi k MSE[ j] 1 j 5. The stadard error associated to each simulated characteristic is placed close to it ad betwee parethesis.

17 SEMI-PARAMETRIC ESTIMATORS 209 Table 1 : Simulated mea values of j 0 1 j 5 for a Fréchet paret. E[ 1 0 ] E[ 2 0 ] E[ 3 0 ] E[ 4 0 ] E[ 5 0 ] Table 2 : Simulated Mea Square Errors of j 0 1 j 5 for a Fréchet paret. MSE[ 1 0 ] MSE[ 2 0 ] MSE[ 3 0 ] MSE[ 4 0 ] MSE[ 5 0 ] I the Tables 3 ad 4 we preset the distributioal behaviour of the above metioed estimators for a Burr model with γ = 1 ad for values = Some fial remarks: 1. The choice of the tuig parameters θ 1 θ 2 seems to be ucotroversial: the pair θ 1 θ 2 = 2 3 seems to be the most coveiet. The tuig parameter ca be ay real positive umber but the value = 1 is the easiest choice maily due to the fact that the computatio of a estimate for a give perhaps large data set is much less time-cosumig wheever we work with M for positive iteger. The choice of τ is more ope ad depeds obviously o the model. This gives a higher flexibility to the choice of the adequate estimator of withi the class of estimators herewith studied. 2. Ideed the most iterestig feature of this class of estimators is the fact that the cosideratio of the sample paths 123τ k as a fuctio of k for large k ad for a few values of τ like τ = ad 2 eables us to idetify easily the most stable sample path ad to get a estimate of.

18 210 M.I. FRAGA ALVES M. IVETTE GOMES ad LAURENS DE HAAN Table 3 : Simulated Mea Values of j 0 1 j 5 for a Burr paret. E[ 1 0 ] E[ 2 0 ] E[ 3 0 ] E[ 4 0 ] E[ 5 0 ] = = = = I Gomes ad Martis [13] the choice of the level k 1 := mi 1 [2/ l l ] led to quite ice results ot a log way from the oes got for the optimal k 0 which is presetly still ideal i the sese that we do ot have yet a practical adequate way of estimatig the optimal sample fractio to be take i the estimatio of made through the semi-parametric estimators we have bee discussig.

19 SEMI-PARAMETRIC ESTIMATORS 211 Table 4 : Simulated Mea Square Errors of j 0 1 j 5 for a Burr paret. MSE[ 1 0 ] MSE[ 2 0 ] MSE[ 3 0 ] MSE[ 4 0 ] MSE[ 5 0 ] = = = = So the simulated behaviour of the estimators at the optimal level herewith preseted has ot yet ay applicatio i practice. These results merely exhibit the optimal MSE we may obtai should we be able to choose k i a optimal way.

20 212 M.I. FRAGA ALVES M. IVETTE GOMES ad LAURENS DE HAAN REFERENCES [1] Beirlat J.; Vyckier P. ad Teugels J.L. Excess fuctio ad estimatio of the extreme-value idex Beroulli [2] Beirlat J.; Vyckier P. ad Teugels J.L. Tail idex estimatio Pareto quatile plots ad regressio diagostics J. Amer. Statist. Assoc [3] Beirlat J.; Dierckx G.; Goegebeur Y. ad Matthys G. Tail idex estimatio ad a expoetial regressio model Extremes 2: [4] Daielsso J.; Haa L. de; Peg L. ad Vries C.G. de Usig a bootstrap method to choose the sample fractio i the tail idex estimatio J. Multivariate Aalysis [5] Dekkers A.L.M.; Eimahl J.H.J. ad Haa L. de A momet estimator for the idex of a extreme-value distributio A. Statist [6] Draisma G.; Haa L. de; Peg L. ad Pereira T.T. A bootstrap-based method to achieve optimality i estimatig the extreme value idex Extremes 2: [7] Drees H. ad Kaufma E. Selectig the optimal sample fractio i uivariate extreme value estimatio Stoch. Proc. ad Appl [8] Feuerverger A. ad Hall P. Estimatig a tail expoet by modellig departure from a Pareto distributio A. Statist [9] Galambos J. The Asymptotic Theory of Extreme Order Statistics 2d editio Krieger [10] Gedeko B.V. Sur la distributio limite du terme maximum d ue série aléatoire A. Math [11] Geluk J. ad Haa L. de Regular Variatio Extesios ad Tauberia Theorems CWI Tract 40 Ceter for Mathematics ad Computer Sciece Amsterdam Netherlads [12] Gomes M.I. ad Martis M.J. Geeralizatios of the Hill estimator asymptotic versus fiite sample behaviour J. Statist. Plaig ad Iferece [13] Gomes M.I. ad Martis M.J. Geeralized Jackkife estimators of the tail idex based o the estimatio of the secod order parameter Notas e Comuicações CEAUL 9/2001 Submitted. [14] Gomes M.I.; de Haa L. ad Peg L. Semi-parametric estimatio of the secod order parameter asymptotic ad fiite sample behaviour Notas e Comuicações CEAUL 8/2000 To appear i Extremes. [15] Gomes M.I.; Martis M.J. ad Neves M. Alteratives to a semi-parametric estimator of parameters of rare evets the Jackkife methodology Extremes 3: [16] Guillou A. ad Hall P. A diagostic for selectig the threshold i extreme value aalysis J.R. Statist. Soc. B [17] Haa L. de O Regular Variatio ad its Applicatio to the Weak Covergece of Sample Extremes Mathematical Cetre Tract 32 Amsterdam [18] Haa L. de ad Stadtmüller U. Geeralized regular variatio of secod order J. Austral. Math. Soc. A

21 SEMI-PARAMETRIC ESTIMATORS 213 [19] Hall P. ad Welsh A.H. Adaptive estimates of parameters of regular variatio A. Statist [20] Hill B.M. A simple geeral approach to iferece about the tail of a distributio A. Statist [21] Leadbetter M.R. ad Nadagopala S. O exceedace poit processes for statioary sequeces uder mild oscillatio restrictios. I Extreme Value Theory J. Hüsler ad R.-D. Reiss Eds. Spriger-Verlag 1989 pp M.I. Fraga Alves ad M. Ivette Gomes D.E.I.O. ad C.E.A.U.L. Uiversity of Lisbo ad Laures de Haa Erasmus Uiversity Rotterdam