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The distribution and parameters of the distribution of the quotient of random variables

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A C T A U N I V E R S I T A T I S L O D Z I E N S I S

F O LIA O EC O N O M I CA 141, 1997

Andrzej Czajkowski, Dariusz Parys*

T H E D IST R IB U T IO N A N D PA R A M ET ER S O F T H E D ISTR IB U TIO N

O F T H E Q U O TIEN T O F R A N D O M VA RIABLES

Abstract. This paper presents some im p o rtan t earlier results o f the research

concerning the properties o f the distribution o f the quotient o f random variables. We present also our own ideas and results o f the research concerning the mean and the variance o f the distribution o f the quotient o f random quadratic forms. Key words: quotient o f random variables, random quadratic forms, m om ent generating function.

***

In this paper we present the m ost im portant results concerning the distribution and the param eters o f the distribution o f the quotient of random variables.

O ur results concerning the power of the random quadratic forms are also outlined. The research aimed at fixing the form o f the distribution of the quotient o f random variables and what had been done so far was assessed as insuficient. M ost o f the basic results were achieved in the m iddle o f this century and the num ber o f papers published in recent years is relatively small.

The m ain reason for this situation are difficulties which arise during estimating the density function (or distribution function) of a given distribution or its basic param eters (mainly m ean and variance). Form ulated theorem s require assum ptions concerning random variables in the num erator and the denom inator (about the form o f the distributions or independence of

variables). ^

Let us consider random variable Z = —; where X , Y are random Л

variables and X Ф 0. It is well know n that the density function of variable Z is of the form:

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9(z) = J f ( x , z x)/ x/dx (I)

— 00

where / ( x , zx) is the density function of bivariate random variable (X , Y) for X = X and Y = Z X .

I f random variables X and Y — Z X are independent then:

9 ( z ) = í \ x \ f i ( x ) f 2(xz)dx ( ľ )

— 00

where / {(х) and f 2(xz) are the density functions of X and Y, respectively. Assume th at random variables X and Y are independent and both have norm al distributions with m ean equal 0 and variance equal 1.

Because the distribution is symmetric the density function has the from:

+ °° 1 +c0 Г 1 a 2 4- o 2z 2~\ g(z) = 2 J xf (x , xz)dx = — — - J x e x p \dz = 0 l l ffxa y 0 L axa y J 1 a]a\ _ 1 Ъ.

(

2

)

П axay a 2 + a \ z 2 П + ^

and the distribution function o f the quotient has the following form:

F W - i + i a r c t g ^ (3)

T hen, the distribution o f the quotient o f random variables when the variables have the standardized normal distributions is the Cauchy distribution and has no m om ent o f any degree.

In 1930 G e a r y introduced form ulas for two independent random

X

variables with norm al distributions. Let be the quotient o f two independent random variables i.e. X ~ N ( n 1, f f l) and У ~ N ( n 2, a 2) and random variable Y in the denom inator is positive. Then the density function o f random variable W is o f the form:

m = - Ľ + W * J 2 exp { — \ (4)

V m W + o l w 2) 3' 2 * 1 2

M . G r e e n (1965), allowing the same assum ptions (see G e a r y 1930) ab o u t norm ality and independence o f the variables from the num erator and

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the denom inator and additionally assuming that the coefficient of variability o f variable Y is n ot too large (in practice not greater then 1/3, has found

/ X \ ( X

estimates o f the m ean J5JÍ — J and the variance D2

\ r 1 , T / 2 , T T / 4 - 1

Е [ у ) * ц 2 [1 + У 2 + Щ ] (5)

D 2( XV) « °}2 [1 + 3 F / + 15V2 ] + ( * V 2[l + 8 V 2] (6)

Y J Н Г 2J Pi

a 2

where V2 = — is the coefficient o f variability o f random variable Y. P-2

M. G reen showed further that form ulas (5) and (6) are approxim ations for - < V2 < - and exact values for V2 < ~ .

The above results have been obtained with the assumption of independence o f random variables X and Y.

R. C. Feller (see F i e l l e r 1993) has found form ulas for the distribution function and the density function of the quotient of random variables with bivariate binorm al distributions w ithout the assum ption o f independence of variables from the nom inator and the denom inator.

The probability P ( W ^ w) has the form:

+ 00+00+00+00 I С 1 1 р ( и , > “ ) = !

! А].

5й7Г7 - * Ь - п ? - (х,- ^ л+

+ y l ) \ d x l d y l (7) where h = * a r (a2 — 2rwoxay -(- w2ct2) 112 ra — wa

P ~ , 2— л— --- j 2\ТУ? > where r is the correlation coefficient,

(ffy — 2 rwaxay + w^tr,)1' 2 x X . = — y — wx У i = («г2 — 2rwax(Ty + w2<t2) 112 '

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In further papers some generalizations of the above results can be found. J . K otlarski considers m ore general case o f the quotient of two random variables of the form (see K o t l a r s k i 1960):

(

8

)

where X u X 2 are independent random variables with the gamma distributions;

9 i . Q 2 e R { ° }

-J. K otlarski shows that for all pairs (qu q 2) i.e. the param eters o f the distributions o f X x and X 2, there exist independent positive random variables and Y2 with distribution essentially different from X \ i , Х \ г ,

Y

such th at the quotient —г has the same distribution as y. 2

The next generalizations were found by G. M arsaglia (see M a r s a g 1 i a 1965). H e considers the quotient o f random variables:

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b + Y K '

where X , Y are independent random variables X ~ N(0, 1), Y ~ N ( 0, 1),

a, b are nonnegative constans.

He shows further that the distribution function F(t) o f random variable

W o f the form:

а д = р[ £ Н

<io>

can be expressed by the bivariate norm al distribution or by m eans o f the N icholson function: . Г а — bí , t 1 r Г — a + bt , t (,) Ш ? ' ~ ■ Т Т Т Н L t t t ? ' • 7 ^ lb,-a)/ST? ь ( b t - a t F(t) =

j

(p(x)dx + ]p(x)dx + 2L[ ---

,

b, - = = 0 0 W l + i VI + 1 I 1 i -лт/ a b + a t \

= ž+n

' +2l\7

77? •

7Г+?) “ mb’ a)

where L(/j, k, <p) = P(£ > h, tj > k) and £ are norm al standardized variables with covariance p, V is the Nicholson function o f the form:

h qx/h

V(h, g) — J J q( x)q(y)dydx, where ę is the density function o f the о о

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F o r large b the second and the third com ponent o f (12) m ay be replaced by 0.5 and 0, hence

а л- X 1 («-«олД+Р <dt-e)/,/r+?

~ b + Y < J S + ^ <P(x)dx = j cp(x)dx (14)

This form ula is a very good approxim ation for estimating values o f the distribution function F(t). The density function o f variable (9) is of the form:

F m = n <15>

where

b + aw

q

V 1 + w

The density function (15) is unim odal or bim odal depending on the values o f a and b. If a > 2,257 then the density is bim odal, but one m axim um is insiqnificant.

In the same paper G. M arsaglia shows the form ula for the distribution function o f the quotient of the sum o f independent random variables with the uniform distribution on interval <0, 1):

P [~“ i + . » » + - • + » . < l j V a W - . h m

Lvi + v2 + — + vm

J

L

s / a 2m + n

J

where (p is the distribution function of the norm al distribution with m ean 0,5(n + /na) and variance (a2m + n)1/2.

A p proxim ate form ulas for the case w here E ( X ) and D 2( X ) are know n can be found in the literature of the field. In particular when Z = ~ , we have:

Л

K * K

(i7>

D2(i)v *

(18>

where: и = E ( X ) ^ 0 and ctz — D 2(X).

The approxim ate form ulas cannot be used in some cases, for example, when random variable X has the norm al distribution ( /( x ) is the den-sity-function o f the variable X ) the m ean e ( ~ j does not exist.

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The authors (see P i e t e o l d et al. 1974) suggest assuming in this case, th a t the variable under consideration has the truncated norm al distribution. In this paper the authors show the approxim ate form ulas to calculate the m ean and the variance o f random variables — with

X

truncated norm al distribution. W ith the use o f published tables the value o f param eters can be obtained. The authors suggest assum ing the so called critical truncation level

= ) - 1 (for ц > 2) (19)

because, in practice, there are no situations where we have no clear conditions at what level to fix the truncation.

If Ц < 2, then the truncation level m ust be choosen only on the grounds o f the essential conditions.

J. R. M a g n u s (1986) presents im portant results concerning the integral representation o f the m om ent generation function and s-th m om ent of quadratic form s and the ratio of these forms (see M i l o and P a r y s 1989).

Let x be а и x 1 vector with the norm al distribution with m ean ц and positively defined covariance m atrix Q = L Ľ . Let A be a n x n symetrie m atrix. Then:

E ( x ' A x y = £ y,(v) П { tr( Ľ A Ľ ) J + j y ! Ľ ~ 1( Ľ A Ľ ) JL~ ' u } n] (20) j= i

where the summing is over all 1 x s vectors v = (nu ..., n,) with coordinates fulfilling the conditions:

£ jnj = S and y, = s ß ' f l K . W ' ] -1 (21)

i j= i

Let and W2 be random variables and P(Ww > 0) = 1. Assum e that the jo in t m om ent generation function exist for Wt and W2 and has the form:

0 ( 0 1, © 2) = ^ [ e x p i © ! ^ + ® 2W2)} (22) for all I©!I < e and &2 < e, в > 0.

Then (see M a r s a g l i a 1965) for seiV the following relation holds:

-°Ц

а

<23)

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Assume that x a n x 1 vector with the norm al distribution with m ean

H and positively defined covariance m atrix П = L Ľ .

Let A be a n x n symetrie quadratic m atrix and B — n x n quadratic m atrix half positively defined. Assume that P is a n x n quadratic orthogonal m atrix such that:

P'L'BLP = D

where P'P = J„ and D is a n x n quadratic diagonal m atrix.

Define A* = P'L'ALP, fi* = P ' L ~ in and A = (In + 2 tD) ~112, R = A A* A,

£ = A/i* where ę is a nx vector.

Then:

\_x'Bx (s ~ l ) ! iT 7( v ) J r 1|A |exp(

(24)

П ( t r # +

X ' R JO njdt

where the summing is over all 1 x s vectors v = (n1(..., n3) with coordinates fulfilling the conditions (21)

and

d = e x p ^ ^ / / i r v j

-These results allowed us to form ulate some conclusions concerning the m om ents of random quadratic forms, their ratios and m odifications. From (20) we have:

E( x'Ax) = tr Ľ A L + ц'Ац

E ( x ' A x ) 2 = ( t r Ľ A Ľ ) 2 + 2 t r ( Ľ A Ľ ) 2 (25)

D \ x ’A x ) = 2tr(L 'A L )2 + 4 ц'A L L Aß

I t’s easy to verify that for ц = 0 above conclusions have simply from:

E( x'Ax) = tr Ľ A L

E ( x 'A x ) 2 = I r ( Ľ A Ľ ) 2 + 2tr ( Ľ A L ) 2 (26)

D \ x ' A x ) = 2tr ( Ľ A Ľ ) 2

In the case ц = 0 we have obtained some results concerning independence o f random quadratic forms.

It is well known that in the case of the independence o f random

Y / y\ E( Y)

variables — and X the equality E[ — ) = is true.

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x 1 A.x

Then from (25) and from the independence o f - and x ' B x we have x Bx

_ f x ' A x \ it L ' AL

( x 'B x / XxL'BL (27)

(x'A x)2

(x1 A x ) 2 + x ' B x

I f у - г л л~2Г:~_~>п anc* (x '/lx )2 + x' Bx are independent then

J i x ' A x ) 2 \ (tr L ' AL ) 2 + 2tr (L' AL) 2

у(х'Л х)2 + x'Bx J (tr L ' AL) 2 -I- 2tr (L' AL) 2 + tr L' BL

From (24) for s = 1 we have obtained the following results:

* [ £ ] = á j t | A | e x p ^ ' ^ ( t r Ä + t 'R Od t. (29)

The properties of the variance imply the following formulas:

D2[xlx]=34 |A|eXP( H (tri? + ™ (tri?2 + 2^ 2^ - ( £

x ' A x x ' B x

(30) We intend to continue our studies in the field of the quotient o f random quadratic forms. In particular we will use them to find some properties of the distribution and param eters of different types of quotient (28) concerning the problem o f regularizing estim ator (see M i l o , P a r y s 1989).

R E FE R E N C E S

F i e l l e r , B. A. (1993): The distribution o f the Index in Norm al Bivariate Population, „Bio- m etrica” , p. 428-440.

G e a r y , R . C. (1930): The Frequency Distribution o f the Quotient o f Two Norm al Variates, „Journal o f the R oyal Statistical Society” , p. 442-446.

G r e e n , M . (1965): The parameters o f the Distribution o f the Quotient o f Two Normal

Variables, „G row th Project Paper” 238, C am bridge.

K o t l a r s k i , J . (1962): On Pairs o f Independent Random Variables Whose Quotient Follows

Som e Known Distribution, „Collegium M athem aticum ” , Vol. IX, p. 151.

K o t l a r s k i , J. (1960): On Random Variables Whose Quotient Follows the Cauchy Law, „Collegium M athem aticum ” , Vol. VII.

M a g n u s , J . R. (1986): The Exact M om ents o f a Ratio o f Quadratic Forms in Norm al

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M a r s a g l i a , G . ( 1966): Ratios o f Norm al Variables and Ratios o f Sum s o f Uniform Variables, „A m erican Statistical A ssociation Journal” , Vol. 60, p . 195-204.

M i l o , W. , P a r y s , D . (1989): Całkowe przedstawienia skalarnych fu n kcji macierzy losowych

i ich zastosowania, Cz. 1 (Estymacja modeli losowych), CPBP 10.09.III.6, Ł ódź (maszynopis

powielony).

N i c h o l s o n , С . (1943): The Probability Integral fo r Two Variables, „Biom etrica” .

P i e t с о 1 d, K ., T o m a s z e w i c z , A., Ż ó ł t o w s k a , E . (1974): M om enty odwrotności zmiennej

losowej o uciętym rozkładzie normalnym, „Przegląd Statystyczny” , z. 4.

Andrzej Czajkowski, Dariusz Parys

R O Z K Ł A D I P A R A M E T R Y R O Z K Ł A D U IL O R A Z U Z M IE N N Y C H LOSOW YCH

W pracy tej prezentujem y wcześniejsze rezultaty badań dotyczących własności rozkładów ilorazów zmiennych losowych.

Ponadto prezentujem y własne wyniki badań dotyczące m om entów rozkładów ilorazów losowych form kw adratow ych.

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