A Quasi Aradhana Distribution with Properties and …A Quasi Aradhana Distribution with Properties...
Transcript of A Quasi Aradhana Distribution with Properties and …A Quasi Aradhana Distribution with Properties...
International Journal of Statistics and Systems
ISSN 0973-2675 Volume 13, Number 1 (2018), pp. 61-80
© Research India Publications
http://www.ripublication.com
A Quasi Aradhana Distribution with Properties and
Application
Rama Shanker & Kamlesh Kumar Shukla
Department of Statistics, Eritrea Institute of Technology, Asmara, Eritrea, India.
Ravi Shanker
Department of Mathematics, G.L.A. College, N.P University, Daltonganj, Jharkhand, India.
Abstract
A two-parameter quasi Aradhana distribution (QAD) which includes one
parameter Aradhana distribution introduced by Shanker (2016) as special case
has been proposed and investigated. Its moments based measures, reliability
properties, stochastic ordering, mean deviations, Bonferroni and Lorenz
curves, order statistics, Renyi entropy measure and stress-strength reliability
have been discussed. For estimating its parameters, the method of moments
and the method of maximum likelihood have also been discussed. Goodness of
fit of QAD has been discussed with a real lifetime data from engineering and
compared with other one parameter and two-parameter lifetime distributions.
Keywords: Aradhana distribution, Moments, Reliability measures, ,
Stochastic ordering, Mean Deviation, Bonferroni and Lorenz curves, Order
Statistics, Renyi entropy measure Stress-strength reliability, Estimation of
parameters, goodness of fit
1. INTRODUCTION
Shanker (2016) introduced a one parameter lifetime distribution named Aradhana
distribution for modeling lifetime data from biomedical sciences and engineering
defined by its probability density function (pdf) and cumulative distribution function
(cdf)
3
2
1 2; 1 ; 0, 0
2 2
xf x x e x
. (1.1)
62 Rama Shanker, Kamlesh Kumar Shukla, Ravi Shanker
1 2
2 2; 1 1 ; 0, 0
2 2
xx xF x e x
(1.2)
It has been shown by Shanker (2016) that in majority of real lifetime datasets it gives
better fit than exponential, Lindley (1958), Akash and Shanker distributions
introduced by Shanker (2015 a, 2015 b). The Aradhana distribution is a three-
component mixture of exponential , a gamma 2, and a gamma
3, distributions with their mixing proportions2
2 2 2
,
2
2
2 2
and
2
2
2 2 respectively.
The first four moments about origin of Ardadhana distribution (1.1) obtained by
Shanker (2016) are
2
1 2
4 6
2 2
,
2
2 2 2
2 6 12
2 2
,
2
3 3 2
6 8 20
2 2
,
2
4 4 2
24 10 30
2 2
The central moments of Aradhana distribution obtained by Shanker (2016) are given
by
4 3 2
2 22 2
8 24 24 12
2 2
6 5 4 3 2
3 33 2
2 12 54 100 108 72 24
2 2
8 7 6 5 4 3 2
4 44 2
3 3 48 304 944 1816 2304 1920 960 240
2 2
Important mathematical and statistical properties of Aradhana distribution including
shapes for varying values of parameter, hazard rate function, mean residual life
function, stochastic ordering, mean deviations, distribution of order Statistics,
Bonferroni and Lorenz curves, Renyi entropy measure and stress-strength reliability
have been discussed by Shanker (2016). Shanker (2016) has also discussed the
A Quasi Aradhana Distribution with Properties and Application 63
estimation of parameter and applications of Aradhana distribution for modeling
lifetime data. Shanker (2017) has obtained a Poisson mixture of Aradhana
distribution and named, ‘Poisson-Aradhana distribution’, discussed its various
statistical properties and estimation of parameter using both the method of moments
and the maximum likelihood estimation as well as applications to model count data
from different fields of knowledge.
In this paper, a two - parameter quasi Aradhana distribution (QAD), of which one
parameter Aradhana distribution introduced by Shanker (2016) is a particular case,
has been proposed. Its raw moments and central moments have been obtained and
coefficients of variation, skewness, kurtosis and index of dispersion have been
discussed. Its hazard rate function, mean residual life function, stochastic ordering,
mean deviations, Bonferroni and Lorenz curves, order statistics, Renyi entropy
measure and stress-strength reliability have been discussed. Both the method of
moments and the method of maximum likelihood have been discussed for estimating
the parameters of QAD. The goodness of fit of the QAD has been presented with a
real lifetime dataset and the fit has been compared with other well known one
parameter and two-parameter lifetime distributions.
2. A QUASI ARADHANA DISTRIBUTION
A two - parameter quasi Aradhana distribution (QAD) having parameters and is
defined by its pdf
2 2
2 2; , ; 0, 0, 2 2 0.
2 2
xf x x e x
(2.1)
It can be easily verified that at , (2.1) reduces to the Aradhana distribution (1.1)
and at 0 , it reduces to gamma 3, distribution. It can be easily verified that
QAD (2.1), like Aradhana distribution, is also a convex combination of exponential
gamma 2, and gamma 3, distributions. We have
2 1 1 2 2 1 2 3; , ; ; ,2 1 ; ,3f x p g x p g x p p g x (2.2)
where 2
1 22 2
2,
2 2 2 2p p
,
1 ; ; 0, 0xg x e x
22 1
2 ; , 2 ; 0, 02
xg x e x x
33 1
3 ; ,3 ; 0, 03
xg x e x x
.
64 Rama Shanker, Kamlesh Kumar Shukla, Ravi Shanker
The corresponding cdf of QAD (2.1) can be obtained as
2
2 2
2 0,
2; , 1 1 ; 0, 02 2
2 2
xx xF x xe
(2.3)
Behaviors of the pdf and the cdf of QAD for varying values of parameters are
presented in figures1 and 2.It is clear from the graphs of the pdf of QAD that it takes
different shapes for varying values of parameters and ranging from positively
skewed to monotonically decreasing and hence QAD can be used for modeling real
lifetime data of various nature.
Figure.1. Behaviors of the pdf of QAD for varying values of parameters and
A Quasi Aradhana Distribution with Properties and Application 65
Figure. 2. Behaviors of the cdf of QAD for varying values of parameters and
3. STATISTICAL CONSTANTS
The r th moment about origin of QAD can be obtained as
2
2
! 2 1 1 2; 1,2,3,...
2 2r r
r r r rr
(3.1)
Thus the first four moments about origin of QAD are given by
2
1 2
4 6
2 2
,
2
2 2 2
2 6 12
2 2
,
2
3 3 2
6 8 20
2 2
,
2
4 4 2
24 10 30
2 2
66 Rama Shanker, Kamlesh Kumar Shukla, Ravi Shanker
Using the relationship between central moments and moments about mean, the central
moments of QAD are thus obtained as
4 3 2
2 22 2
8 24 24 12
2 2
6 5 4 3 2
3 33 2
2 12 54 100 108 72 24
2 2
8 7 6 5 4 3 2
4 44 2
3 3 48 304 944 1816 2304 1920 960 240
2 2
The coefficient of variation .C V , coefficient of skewness 1 , coefficient of
kurtosis 2 and index of dispersion of QAD are obtained as
4 3 2
2
1
8 24 24 12.
4 6C V
6 5 4 3 2
31 3 23/2 4 3 2
2
2 12 54 100 108 72 24
8 24 24 12
8 7 6 5 4 3 2
42 22 4 3 2
2
3 3 48 304 944 1816 2304 1920 960 240
8 24 24 12
2 4 3 2
2 2
1
8 24 24 12
2 2 4 6
.
One of the interesting features of QAD is that the expressions for C.V, 1 and
2 depends only on the parameter and whereas depends on the parameter and
both. The graphs of C.V, 1 , 2 , and of QAD have been drawn for varying
values of the parameters and presented in figure 3. It is obvious from the graph of
coefficient of variation that it is slowly increasing for increased values of the
A Quasi Aradhana Distribution with Properties and Application 67
parameter and slowly it reaches towards the constant value of 1. The coefficient of
skewness starts increasing for increased value of the parameter but once the value
of is greater than 2, it starts decreasing. The coefficient of kurtosis is always
increasing for increased value of the parameter . If the value of is less than 2 and
increases, index of dispersion is monotonically decreasing and its shape is concave
upward. If the value of is less than 1 and 3 , the index of dispersion is
increasing and as 3 , it starts decreasing and its shape is concave downward.
Figure.3. Behaviors of C.V, 1 , 2 , and of QAD for varying values of
parameters and
68 Rama Shanker, Kamlesh Kumar Shukla, Ravi Shanker
4. STOCHASTIC ORDERING
Stochastic ordering of positive continuous random variables is an important tool for
judging their comparative behavior. A random variable X is said to be smaller than a
random variable Y in the
(i) stochastic order stX Y if X YF x F x for all x
(ii) hazard rate order hrX Y if X Yh x h x for all x
(iii) mean residual life order mrlX Y if X Ym x m x for all x
(iv) likelihood ratio order lrX Y if
X
Y
f xf x
decreases in x .
The following results due to Shaked and Shanthikumar (1994) are well known for
establishing stochastic ordering of distributions
lr hr mrlX Y X Y X Y
stX Y
The QAD is ordered with respect to the strongest ‘likelihood ratio ordering’ as
established in the following theorem:
Theorem: Let X QAD 1 1, and Y QAD 2 2, . If 1 2 1 2and or
1 2 1 2and , then lrX Y and hence hrX Y , mrlX Y and stX Y .
Proof: We have
1 1 1 2
2 2
22
1 2 2; , 1 1
; , 22 22 1 1
2 2
2 2
X
Y
f x xf x
x ex
; 0x
Now
1 1
2 2
2
1 2 2; , 1 11 2; , 2
2 22 1 1
2 2ln ln 2ln
2 2
X
Y
f xf x
x xx
.
This gives
1 1
2 2
; , 1 2 2 1
1 2; ,
1 1 2 2
2ln X
Y
f xf x
ddx x x
.
Thus if 1 2 1 2and or 1 2 1 2and ,
1 1
2 2
; ,
; ,ln 0X
Y
f xf x
ddx
. This means that
lrX Y and hence hrX Y , mrlX Y and stX Y . This shows flexibility of QAD
over Aradhana distribution introduced by Shanker (2016).
A Quasi Aradhana Distribution with Properties and Application 69
5. MEAN DEVIATIONS
The amount of variation in a population is generally measured to some extent by the
totality of deviations usually either from the mean or the median. These are known as
the mean deviation about the mean and the mean deviation about the median and are
defined as
1
0
X x f x dx
and 2
0
X x M f x dx
, respectively, where
E X and Median M X . The measures 1 X and 2 X can be
calculated using the following simplified relationships
1
0
X x f x dx x f x dx
0
1F x f x dx F x f x dx
2 2 2F x f x dx
0
2 2F x f x dx
(5.1)
and
2
0
M
M
X M x f x dx x M f x dx
0
1
M
M
M F M x f x dx M F M x f x dx
2M
x f x dx
0
2
M
x f x dx (5.2)
Using pdf (2.1) and the mean of QAD, we get
3 3 2 2 2 2
2 2
0
2 3 4 6 4 6; ,
2 2
ex f x dx
(5.3)
70 Rama Shanker, Kamlesh Kumar Shukla, Ravi Shanker
3 3 2 2 2 2
2 2
0
2 3 4 6 4 6; ,
2 2
MM M M M ex f x dx
(5.4)
Using expressions from (5.1), (5.2), (5.3), and (5.4), the mean deviation about
mean, 1 X and the mean deviation about median, 2 X of QAD are obtained as
2 2 2
1 2
2 2 2 4 6
2 2
eX
(5.5)
3 3 2 2 2 2
2 2
2 2 3 4 6 4 6
2 2
MM M M eX
(5.6)
6. BONFERRONI AND LORENZ CURVES
The Bonferroni and Lorenz curves ( Bonferroni, 1930) and Bonferroni and Gini
indices have applications not only in economics to study the variation in income and
poverty, but also in other fields like reliability, demography, insurance and medicine.
The Bonferroni and Lorenz curves are defined as
0 0
1 1 1q
q q
B p x f x dx x f x dx x f x dx x f x dxp p p
(6.1)
and 0 0
1 1 1q
q q
L p x f x dx x f x dx x f x dx x f x dx
(6.2)
respectively or equivalently
1
0
1p
B p F x dxp
(6.3)
and 1
0
1p
L p F x dx
(6.4)
respectively, where E X and 1q F p .
A Quasi Aradhana Distribution with Properties and Application 71
The Bonferroni and Gini indices are thus defined as
1
0
1B B p dp (6.5)
and 1
0
1 2G L p dp (6.6)
respectively.
Using pdf of QAD (2.1), we get
3 3 2 2 2 2
2 2
2 3 4 6 4 6; ,
2 2
q
q
q q q ex f x dx
(6.7)
Now using equation (6.7) in (6.1) and (6.2), we get
3 3 2 2 2 2
2
2 3 4 6 4 611
4 6
qq q q eB p
p
(6.8)
and 3 3 2 2 2 2
2
2 3 4 6 4 61
4 6
qq q q eL p
(6.9)
Now using equations (6.8) and (6.9) in (6.5) and (6.6), the Bonferroni and Gini
indices of QAD are thus obtained as
3 3 2 2 2 2
2
2 3 4 6 4 61
4 6
qq q q eB
(6.10)
3 3 2 2 2 2
2
2 2 3 4 6 4 61
4 6
qq q q eG
(6.11)
7. RELIABILITY MEASURES
There are three important reliability measures namely hazard rate function, mean
residual life function and stress-strength reliability and each will be discussed in this
section.
7.1. Hazard Rate Function and Mean Residual Life Function
Let X be a continuous random variable with pdf f x and cdf F x . The hazard rate
72 Rama Shanker, Kamlesh Kumar Shukla, Ravi Shanker
function (also known as the failure rate function) and the mean residual life function
of X are respectively defined as
0lim
1x
P X x x X x f xh x
x F x
(7.1.1)
and
1
11 x
m x E X x X x F t dtF x
(7.1.2)
The corresponding hazard rate function, h x and the mean residual life
function, m x of QAD (2.1) are obtained as
2
22 2 2 2
xh x
x x
(7.1.3)
and
2
2
12 2 2 2
2 2 2 2
tx
x
m x t t e dtx x e
2 2 2
2
2 2 4 6
2 2 2 2
x x
x x
(7.1.4)
It can be easily verified that 2
20 0
2 2h f
and
2
12
4 60
2 2m
. The graphs of h x and m x of QAD for varying
values of parameters are shown in figure 4. The graphs of h x is monotonically
increasing whereas the graphs of m x is monotonically decreasing.
A Quasi Aradhana Distribution with Properties and Application 73
Figure. 4. Graphs of h x and m x of QAD for varying values of
parameters and
7.2. STRESS-STRENGTH RELIABILITY
The stress- strength reliability describes the life of a component which has random
strength X that is subjected to a random stress Y . When the stress applied to it
exceeds the strength, the component fails instantly and the component will function
satisfactorily till X Y . Therefore, R P Y X is a measure of component
reliability and in statistics literature it is known as stress-strength parameter. It has
wide applications in biomedical sciences and engineering.
Let X and Y be independent strength and stress random variables having QAD (2.1)
74 Rama Shanker, Kamlesh Kumar Shukla, Ravi Shanker
with parameter 1 1, and 2 2, respectively. Then the stress-strength reliability
R can be obtained as
0
| XR P Y X P Y X X x f x dx
2 1 1 2 2 2
0
; , ; ,f x F x dx
2 2 2 2
1 2 2 1 2 1 1 2 1 2
22 2 2 2
2 2 1 1 2 1 2 1 2 1 2
1 32 2
1 1 2 2 2 1 2 1 2
42 2
1 2 2 1 2
52 2
1 1 2 2 1 2
24 6 2 1 2
2 2 2 4 1
2 2 2 2 1
2 21
2 2 2 2
.
It can be easily verified that above expression reduces to the corresponding expression
for Aradhana distribution (1.1) at 1 1 and 2 2 .
8. ORDER STATISTICS AND RENYI ENTROPY MEASURE
8.1. Order Statistics
Let 1 2, ,..., nX X X be a random sample of size n from QAD (2.1). Let
1 2... nX X X denote the corresponding order statistics. The pdf and the cdf of
the k th order statistic, say kY X are given by
1!1
1 ! !
n kkY
nf y F y F y f yk n k
1
0
!1
1 ! !
n kl k l
l
n kn F y f ylk n k
and
1n n jj
Yj k
nF y F y F y
j
0
1n jn
l j l
j k l
n n jF y
j l
,
respectively, for 1,2,3,...,k n .
A Quasi Aradhana Distribution with Properties and Application 75
Thus, the pdf and the cdf of k th order statistic of QAD (2.1) are obtained as
2
20
12
2
!1
2 2 1 ! !
2 2 2 21
2 2
x n kl
Yl
k lx
n kn x ef y
lk n k
x x e
and
2
20
2 2 2 21 1
2 2
j lxn jn
lY
j k l
x x en n jF y
j l
8.2. Renyi Entropy Measure
An entropy of a random variable X is a measure of the variation of uncertainty. A
popular entropy measure is Renyi entropy (1961). If X is a continuous random
variable having probability density function .f , then Renyi entropy is defined as
1log
1RT f x dx
where 0 and 1 .
Thus, the Renyi entropy for QAD (2.1) can be obtained as
2
20
1log
1 2 2
xRT x e dx
2
2
20
1log 1
1 2 2
xx e dx
2
200
21log
1 2 2
jx
j
x e dxj
21 1
20 0
21log
1 2 2
j jx j
je x dx
j
76 Rama Shanker, Kamlesh Kumar Shukla, Ravi Shanker
2
12
0
2 11log
1 2 2
j j
jj
jj
12
12
0
2 11log
1 2 2
jj
j
jj
9. ESTIMATION OF PARAMETERS
9.1. Method of Moments Estimates (MOME) of parameters
Since the QAD (2.1) has two parameters to be estimated, using the first two moments
about origin, we get
2 2
2
2 22
1
2 6 12 2 2Say)
4 6k
This gives a fourth degree polynomial equation in as
4 3 22 8 2 4 7 13 24 2 3 12 3 4 0k k k k k (9.1.1)
Replacing the first and second population moments about origin 1 and 2 by their
respective sample moments, an estimate of k can be obtained and substituting the
value of k in equation (9.1.1), the MOME of parameter can be obtained.
Substituting this estimate of in the expression for the mean of QAD (2.1), MOME
of parameter can be obtained as
2
2
4 6
2 2 x
(9.1.2)
9.2. Maximum Likelihood Estimates (MLE) of parameters
Let 1 2, , ... , nx x x be a random sample of size n from QAD (2.1)). The likelihood
function, L of QAD is given by
2
212 2
n nn x
ii
L x e
The natural log likelihood function is thus obtained as
21
ln ln 2 ln2 2
n
ii
L n x n x
A Quasi Aradhana Distribution with Properties and Application 77
The maximum likelihood estimates (MLEs) ˆ ˆ, of , are then the solutions
of the following non-linear equations
1
ln2 0
ni
i i
xL n n xx
21
2 1ln 12 0
2 2
n
i i
nLx
,
where x is the sample mean.
These two natural log likelihood equations do not seem to be solved directly because
they are not in closed form. However, the Fisher’s scoring method can be applied to
solve these equations. We have
22
22 21
ln2
ni
i i
xL nx
2
2 22 21
2 2ln2
2 2
ni
i i
n xLx
2 2
21
ln ln2
ni
i i
xL Lx
The iterative solution of the following equations will be the MLEs ˆ ˆ, of
parameters , of QAD
00
00
2 2
2
0
2 2
0
ˆ2ˆ
ˆˆ
ln ln lnˆ
lnˆln ln
L L L
LL L
where 0 and 0 are the initial values of and , respectively given by the MOME
of the parameters of QAD. These equations are solved iteratively till sufficiently
close values of ̂ and ̂ are obtained.
10. AN ILLUSTRATIVE EXAMPLE
The goodness of fit of QAD has been discussed with a real lifetime dataset from
engineering and the fit has been compared with one parameter and two-parameter
lifetime distributions. The following dataset represents the failure times (in minutes)
78 Rama Shanker, Kamlesh Kumar Shukla, Ravi Shanker
for a sample of 15 electronic components in an accelerated life test, Lawless (2003,
pp. 204)
1.4 5.1 6.3 10.8 12.1 18.5 19.7 22.2 23.0 30.6 37.3 46.3
53.9 59.8 66.2
The pdf and the cdf of the fitted distributions have been given in table 1. Recall that
the quasi Shanker distribution (QSD) has been introduced by Shanker and Shukla
(2017) and the exponentiated exponential distribution (EED) has been introduced by
Gupta and Kundu (1999). The Lindley distribution has been introduced by Lindley
(1958) and its detailed study has been done by Ghitany et al (2008).
In order to compare the considered distributions, values of 2ln L , AIC(Akaike
Information Criterion) and K-S Statistic ( Kolmogorov-Smirnov Statistic) and p-
value for the dataset have been computed and presented in table 2. The AIC and K-S
Statistic has been computed using the following formulae:
2ln 2AIC L k and 0- Sup nx
K S F x F x , where k number of parameters,
n sample size, nF x is the empirical distribution function and 0F x is the
theoretical cumulative distribution function. The best distribution corresponds to the
lower values of 2ln L , AIC, and K-S statistic.
Table 1: The pdf and the cdf of the fitted distributions
Models p.d.f. c.d.f.
QSD
3
3
2
; ,2
x
f x
x x e
2 2
3
2; , 1 1
2
x
xx
F x e
Weibull 1; , xf x x e ; , 1 xF x e
Gamma
1; , xf x x e
,; , 1
xF x
Lognormal
21 log
21; ,
2
x
f x ex
log
; ,xF x
EED 1
; , 1 x xf x e e
; , 1 xF x e
Lindley
2
; 11
xf x x e
1
; 11
xxF x e
Exponential ; xf x e ; 1 xF x e
A Quasi Aradhana Distribution with Properties and Application 79
Table 2. MLE’s, 2ln L , Standard Error (S.E), AIC, K-S Statistics and p-value of the
fitted distributions of dataset.
Distributions ML Estimates S.E 2ln L AIC K-S p-value
QAD ˆ 0.07548 0.02308 128.07 132.07 0.095 0.997
ˆ 1.23504 1.17209
QSD ˆ 0.07389 0.04131 129.37 133.37 0.121 0.961
ˆ 0.00147 0.04401
Gamma ˆ 0.05236 0.02067 128.37 132.37 0.102 0.992
ˆ 1.44219 0.47771
Weibull ˆ 0.01190 0.01124 128.04 132.04 0.098 0.995
ˆ 1.30586 0.24925
Lognormal ˆ 2.93059 0.26472 131.23 135.23 0.161 0.951
ˆ 1.02527 0.18718
EED ˆ 0.04529 0.01372 128.47 132.47 0.108 0.986
ˆ 1.44347 0.51301
Aradhana ˆ 0.10511 0.01567 132.30 134.30 0.172 0.701
Lindley ˆ 0.07022 0.01283 128.81 130.81 0.110 0.983
Exponential ˆ 0.03631 0.00937 129.47 131.47 0.156 0.807
It can be easily observed from table 2 that the QAD gives better fit than one parameter
exponential, Lindley and Aradhana distributions and two-parameter QSD, Gamma,
Weibull lognormal and EED and thus it can be considered as an important two-
parameter lifetime distribution for modeling lifetime dataset from engineering over
these distributions.
11. CONCLUDING REMARKS
In the present paper a two - parameter quasi Aradhana distribution (QAD) which
includes one parameter Aradhana distribution introduced by Shanker (2016) as a
particular case has been proposed and studied. Its statistical properties including
moments, coefficient of variation, skewness, kurtosis, index of dispersion, hazard rate
function, mean residual life function, stochastic ordering, mean deviations, order
statistics, Bonferroni and Lorenz curves, and stress-strength reliability have been
80 Rama Shanker, Kamlesh Kumar Shukla, Ravi Shanker
discussed. The method of moments and the method of maximum likelihood have also
been discussed for estimating its parameters. Finally, a numerical example of real
lifetime dataset from engineering has been presented to test the goodness of fit of the
QAD over one parameter exponential, Lindley and Aradhana , and two-parameter
EED, Lognormal, Gamma, Weibull and QSD.
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