Computational Statistics and Data Analysis 54 (2010) 473–495

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Computational Statistics and Data Analysis
journal homepage: www.elsevier.com/locate/csda

Nonparametric multiplicative bias correction for kernel-type density
estimation on the unit intervalI
Masayuki Hirukawa ∗Department of Economics, Northern Illinois University, Zulauf Hall 515, DeKalb, IL 60115, USA

articleabstract

info

Article history:
Received 31 March 2009
Received in revised form 9 September 2009
Accepted 10 September 2009
Available online 23 September 2009This paper demonstrates that two classes of multiplicative bias correction (MBC)
techniques, originally proposed for density estimation using symmetric second-orderkernels by Terrell and Scott (1980) and Jones et al. (1995), can be applied to density
estimation using the beta and modified beta kernels. It is shown that, under sufficientsmoothness of the true density, both MBC techniques reduce the order of magnitude inbias, whereas the order of magnitude in variance remains unchanged. Accordingly, meansquared errors of these MBC estimators achieve a faster convergence rate of O n−8/9 forthe interior part, when best implemented. Furthermore, the estimators always generate
nonnegative density estimates by construction. To implement the MBC estimators, a plugin smoothing parameter choice method is proposed. Monte Carlo simulations indicate goodfinite sample performance of the estimators.
© 2009 Elsevier B.V. All rights reserved.

1. Introduction
Let X1 , . . . , Xn be a random sample drawn from a univariate distribution with density f having support [0, 1]. In order toestimate the density at a design point x ∈ [0, 1], we may initially consider a usual kernel density estimatornfˆ (x) =1 X
nh i=1K

x − Xih,where K (·) is a symmetric probability density function having support R or some finite interval such as [−1, 1], and h > 0 isa smoothing parameter called the bandwidth. However, this estimator does not work well for density estimation on the unit
interval without boundary correction. Indeed, there is huge literature on correction methods for boundary effects; examples
include Müller (1991), Jones (1993), Jones and Foster (1996), Zhang et al. (1999), Hall and Park (2002), and Karunamuni andAlberts (2005), to name a few.
As an alternative device to these boundary correction methods, Chen (1999) proposes a class of so-called asymmetrickernel functions that have the same support as the density to be estimated has. Specifically, he considers the beta kernel
density estimatornfˆB (x) =

1X
n i=1

KB(x/b+1,(1−x)/b+1) (Xi ) ,I I would like to thank an Associate Editor and two anonymous referees for helpful comments and suggestions.

∗ Tel.: +1 815 753 6434; fax: +1 815 752 1019.
E-mail address: mhirukawa@niu.edu.
URL: http://www.math.niu.edu/∼hirukawa/.

0167-9473/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.csda.2009.09.017

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M. Hirukawa / Computational Statistics and Data Analysis 54 (2010) 473–495

and the modified beta kernel density estimatornfˆMB (x) =1Xn i =1

KB(ρb,0 (x),ρb,1 (x)) (Xi ) ,uα−1 (1 − u)β−1 1 {0 ≤ u ≤ 1} /B (α, β) is the density function for the beta distribution with

where KB(α,β) (u) =parameters α, β > 0,

ρb,0 (x) =ρb,1 (x) =ρb (x)x/b

(1 − x) /b for x ∈ [0, 1 − 2b]ρb (1 − x) for x ∈ (1 − 2b, 1] ,rρb (x) = 2b2 +5

2

for x ∈ [0, 2b)
for x ∈ [2b, 1] ,−4b4 + 6b2 +

9

4

x

− x2 − ,band b > 0 is a smoothing parameter satisfying b + nb → 0 as n → ∞. Because the beta and modified beta kernels aredefined on the unit interval, both fˆB (x) and fˆMB (x) are free of boundary bias. Moreover, the shapes of these kernels vary
according to the design point, and thus the kernels change the amount of smoothing in a natural way.
Strictly speaking, the beta and modified beta kernels should be referred to as kernel-type weighting functions. As
discussed in Gouriéroux and Monfort (2006) and Jones and Henderson (2007a), unlike the cases of symmetric kernels,roles of the data point X and the design point x in these functions are not exchangeable, which causes fˆB (x) and fˆMB (x)not to integrate to unity. The phrase ‘kernel-type’ signifies the fact that they lack a basic property every symmetric kernel
possesses. Nevertheless, following the convention in the literature, these kernel-type functions are simply called the beta
and modified beta kernels throughout.
Smoothing by beta density weights is originally proposed as an extension to Bernstein polynomial smoothing (Brown
and Chen, 1999). After Chen (2000a) apply two beta kernels to nonparametric regression estimation, Bouezmarni and Rolin(2003) demonstrate that fˆB (x) is consistent even if the true density becomes unbounded at a boundary. As an example ofempirical work, Renault and Scaillet (2004) and Hagmann et al. (2005) apply fˆB (x) for estimating recovery rate densities incredit risk modelling. Gouriéroux and Monfort (2006) point out potential issues of applying fˆB (x) in recovery rate density
estimation, and propose their possible solutions.
Provided that the true density f is twice continuously differentiable, the biases of fˆB (x) and fˆMB (x) are O (b) as b → 0. This
paper considers the improvements in beta kernel density estimation that reduce the order of magnitude in bias to O b2
under sufficient differentiability of f , while the order of magnitude in variance is maintained. In the cases of symmetrickernels, this kind of rate improvements can be typically achieved by employing higher-order kernels; see Jones and Foster(1993) for methods of generating higher-order kernels from a given second-order kernel. To our best knowledge, equivalenttechniques are yet to be proposed for asymmetric kernels (including two beta kernels). Instead, this paper applies twoclasses of multiplicative bias correction (MBC) techniques to attain the rate improvements. The first class of MBC concerned
is to construct a multiplicative combination of two density estimators using different smoothing parameters. This idea isoriginally proposed by Terrell and Scott (1980) as an additive bias correction to the logarithms of densities, and later it isgeneralized and reinterpreted as an MBC technique by Koshkin (1988) and Jones and Foster (1993),
The secondn respectively.
oclass of MBC in the spirit of Jones et al. (1995) is based on the idea of expressing f (x) = fˆ (x) f (x) /fˆ (x) and estimating

the bias-correction term f (x) /fˆ (x) nonparametrically. This MBC technique is also applied in nonparametric regression
estimation (Linton and Nielsen, 1994), hazard estimation (Nielsen, 1998; Nielsen and Tanggaard, 2001), and spectral matrix
estimation (Xiao and Linton, 2002; Hirukawa, 2006). When applied to beta kernel density estimation, both MBC techniques
still yield the estimators that are free of boundary bias. In addition, these estimators have a practically appealing property.
They always generate nonnegative density estimates everywhere by construction, as fˆB (x) and fˆMB (x) do. To implement thetwo classes of MBC estimators, this paper also proposes a plug-in method of choosing the smoothing parameter b with abeta density used as a reference.
In the related literature, Hagmann and Scaillet (2007) apply a semi-parametric MBC technique in the spirit of Hjort andJones (1996) (called local multiplicative bias correction, or LMBC) to the beta density estimator. Gustafsson et al. (2009)
propose another semi-parametric MBC technique called local transformation bias correction (LTBC), which basically follows
the idea of Rudemo (1991). In both LMBC and LTBC, the beta kernel is used in nonparametric bias correction for initial
parametric density estimation. A key difference is that the bias correction is made for the original data in LMBC and forthe data transformed on the unit interval in LTBC. However, unlike the MBC estimators proposed in this paper, neither
LMBC nor LTBC improves the bias in order of magnitude. Furthermore, another class of asymmetric kernels, including the
gamma (Chen, 2000b), inverse Gaussian and reciprocal inverse Gaussian (Scaillet, 2004) kernels, has been proposed fordensity estimation with support [0, ∞), yielding density estimators that are free of boundary bias near the origin. Two MBC
techniques considered in this paper can be readily applied for these kernels; research extensions in this direction will be
addressed in a separate paper.

M. Hirukawa / Computational Statistics and Data Analysis 54 (2010) 473–495

475

The remainder of the paper is organized as follows. Section 2 develops asymptotic properties of two classes of MBC
density estimators. Section 3 proposes a plug-in smoothing parameter choice method, and conducts small Monte Carlosimulations to check finite sample properties of MBC estimators. Section 4 applies the MBC estimators for estimating thedensity of eruption durations of the Old Faithful Geyser in Yellowstone National Park, Wyoming, USA. Section 5 summarizes
the main results of the paper. All proofs are given in the Appendix.
This paper adopts the following notational conventions: B (α, β) = 0 (α) 0 (β) /0 (α + β) signifies the beta function;d

R ∞ a−1yexp (−y) dy, a > 0 is the gamma function; and 1 {·} denotes an indicator function. The expression ‘X = Y ’reads ‘‘A random variable X obeys the distribution Y ’’. Lastly, the expression ‘Xn ∼ Yn ’ is used whenever Xn /Yn → 1 asn → ∞.

0 (a) =

0

2. Beta MBC density estimators
2.1. Definitions
The idea of the geometric estimator by Terrell and Scott (1980, abbreviated as ‘‘TS’’ hereafter) can be readily extended
to beta kernel density estimation. For a given kernel j ∈ {B, MB}, let fˆj,b (x) and fˆj,b/c (x) denote density estimators usingsmoothing parameters b and b/c, respectively, where c ∈ (0, 1) is some predetermined constant that does not depend on
the design point x. The TS-MBC beta kernel density estimators are defined aso 1−1 c nnf˜TS,j (x) = fˆj,b (x)fˆj,b/c (x)o− 1−c c.

The other MBC technique in the spirit of Jones n
et al. (1995, abbreviatedas ‘‘JLN’’ hereafter) utilizes a single smoothingoparameter b. In light of the identity f (x) = fˆj,b (x) f (x) /fˆj,b (x) , the JLN-MBC beta kernel density estimators are definedas(f˜JLN,j (x) = fˆj,b (x)1 X K(j;x,b) (Xi )
n

n i=1

fˆj,b (Xi )),where K(j;x,b) (·) denotes the kernel j. Recognize that the term inside the brackets is a natural estimator of the biascorrection term f (x) /fˆj,b (x). Also, by construction, both f˜TS,j (x) and f˜JLN,j (x) always generate nonnegative density estimates
everywhere.
2.2. Asymptotic properties
To approximate the bias and variance of each MBC estimator, the following regularity conditions are assumed:(A1) For a given x ∈ [0, 1], f (x) > 0 and f 0000 (x) is continuous and bounded in the neighborhood of x.
(A2) The smoothing parameter b = bn satisfies b → 0 and nb3 → ∞ as n → ∞.
The smoothness condition for the true density f in (A1) is standard for consistency of density estimators using fourthorder kernels, whereas the condition that f is bounded away from zero is requiredfor MBC. The condition (A2) implies that
the convergence rate of the smoothing parameter b is slower than O n−1/3 . This condition is required to control the orderof magnitude of remainder terms in the approximation to the bias of each MBC
 estimator. It will be shown shortly that themean squared error (MSE)-optimal smoothing parameter is b∗ = O n−2/9 if the design point x satisfies x/b → ∞ and
(1 − x) /b → ∞, and bF = O n−1/5 if x/b → κ or (1 − x) /b → κ for some κ > 0; these convergence rates are indeed

within the required range.
We also call the position of x as ‘‘interior x’’ if x/b → ∞ and (1 − x) /b → ∞, and ‘‘boundary x’’ if x/b → κ or
(1 − x) /b → κ . The following two theorems present the approximations to bias and variance terms of two MBC estimators,
whose proofs are given in the Appendix.
Theorem 1. Suppose that the conditions (A1) and (A2) hold. Then, the biases of f˜TS,B (x) and f˜TS,MB (x) can be approximated by

n

=

1

o

bias f˜TS,B (x) ∼c (1 − c )1" c (1 − c )2p B ( x) b2

1
1 (1 − 2x) f 0 (x) + 2 x (1 − x) f 00 (x)

f (x)

5

1

6

8

2



1
− −2 (1 − 2x) f 0 (x) +11x2 − 11x + 2 f 00 (x)+ x (1 − x) (1 − 2x) f 000 (x) + x2 (1 − x)2 f 0000 (x)

2

#

b2 ,

476

M. Hirukawa / Computational Statistics and Data Analysis 54 (2010) 473–495

for x ∈ [2b, 1 − 2b]pMB (x) b2
× pMB,0 (x) b2 for x ∈ [0, 2b)c (1 − c )pMB,1 (x) b2 for x ∈ (1 − 2b, 1]
"



2
2
1
1 x2 (1 − x) f 00 (x)− − x (1 − x) f 00 (x)


8
f
x
2
(
)



#



1
1 2
2
000
2 0000
 + 3 x (1 − x) (1 − 2x) f (x) + 8 x (1 − x) f (x) b
1
#
"

=
×
2
n

c (1 − c )1 ξb2 (x) f 0 (x)x o 002

f (x) b2
− ξb (x) + ξb (x) +


2
f
x
b
(
)




#
"




2


1 ξb2 (1 − x) f 0 (x)
1−x

00
2

f (x) b2
− ξb (1 − x) + ξb (1 − x) +

2
f (x)
b
n

o
bias f˜TS,MB (x) ∼1for x ∈ [2b, 1 − 2b]for x ∈ [0, 2b)
for x ∈ (1 − 2b, 1] ,whereξb (x) =(1 − x) {ρb (x) − x/b}
= O (1) .
1 + b {ρb (x) − x/b}

The variance of f˜TS,B (x) can be approximated byλ (c )
n−1 b−1/2 f (x)√ √ 2 π x ((1 − x)c κ+1no
12cc
0 ((1 + c ) κ + 1)
0 (2κ + 1)−
v ar f˜TS,B (x) ∼κ2
2κ+1 0 2 (κ + 1)

2
1
+
c
1
+
c
0
(
)
(κ + 1) 0 (c κ + 1)
1
−
c
(
)





0
2c
κ
+
1
(
)

 + c3n−1 b−1 f (x)22c κ+1 0 2 (c κ + 1)
where

λ (c ) =for boundary x,√

1 + c 5/2 (1 + c )1/2 − 2 2c 3/2



for interior x(1 + c )1/2 (1 − c )2.

Theorem 2. Suppose that the conditions (A1) and (A2) hold. Then, the biases of f˜JLN,B (x) and f˜JLN,MB (x) can be approximated bynobias f˜JLN,B (x)∼ qB (x) b2")0(1 − 2x) f 0 (x) + 12 x (1 − x) f 00 (x)= −f (x) (1 − 2x)
f (x)
(
)00 #
(1 − 2x) f 0 (x) + 21 x (1 − x) f 00 (x)1+ x (1 − 2x)b2 ,2f ( x)
for x ∈ [2b, 1 − 2b]qMB (x) b2nobias f˜JLN,MB (x) ∼ qMB,0 (x) b2 for x ∈ [0, 2b)
qMB,1 (x) b2 for x ∈ (1 − 2b, 1]
00


1
x (1 − x) f 00 (x)


−
x
1
−
x
f
x
b2 for x ∈ [2b, 1 − 2b]
(
)
(
)



4
f (x)
ξ (x) f 0 (x) 0 2= −ξb (x) f (x) bbfor x ∈ [0, 2b)f (x)00ξ (1 − x) f (x)−ξb (1 − x) f (x) bb2
for x ∈ (1 − 2b, 1] ,f (x)(where ξb (x) is defined in Theorem 1. The variance of f˜JLN,B (x) can be approximated by1n−1 b−1/2 f (x)
 √ √
o 
2
π
x
(1 − x)v ar f˜JLN,B (x) ∼ 0 (2κ + 1) n−1 b−1 f (x)22κ+1 0 2 (κ + 1)nfor interior x
for boundary x.

M. Hirukawa / Computational Statistics and Data Analysis 54 (2010) 473–495

477

Asymptotic variances of f˜TS,MB (x) and f˜JLN,MB (x) are similar to those of f˜TS,B (x) and f˜JLN,B (x), except that the multipliersin front of n−1 b−1 for boundary x have slightly different forms. Two theorems demonstrate that both TS- and JLN-MBC
estimators are free of boundary bias. More importantly, these two MBC estimators reduce the order of magnitude in bias
from O (b) to O b2 , while their variances are still O n−1 b−1/2 for interior x and O n−1 b−1 for boundary x. We can also see
that, for each of two MBC techniques, the modified beta kernel estimator eliminates the term involving f 0 (x) from the bias
over the interior region. Although f 0 (x) appears in the bias only in two small regions near the boundaries, it is compensatedby disappearance of higher-order derivatives.
The variance of JLN-MBC estimators is first-order asymptotically equivalent to that of bias-uncorrected estimators fˆB (x)and fˆMB (x) for interior x. While the beta LMBC density estimator in Hagmann and Scaillet (2007, Proposition 2) also yields
the same leading variance term, it does not reduce the bias in order of magnitude. In contrast, when JLN-MBC is applied
for density estimation using a symmetric second-order kernel, the leading variance term tends to be larger (although not
inflated in order of magnitude) because the multiplier in the variance term involves the roughness (or squared integral) of
the ‘twiced’ kernel (Stuetzle and Mittal, 1979) that is generated by the original kernel. Moreover, due to the multiplier λ (c ),which is increasing in c ∈ (0, 1) and takes the value from 1 to 27/16, the asymptotic variance of TS-MBC estimators tends
to be larger than those of fˆB (x) and fˆMB (x) for interior x.
For interior x, MSEs of f˜TS,j (x) and f˜JLN,j (x) can be approximated bynp2j (x)on−1 b−1/2 λ (c ) f (x)+ o b4 + n−1 b−1/2 ,√ √2 π x ( 1 − x)
(1 − c )no−1 −1/2
nbf (x)MSE f˜JLN,j (x) = q2j (x) b4 + √ √+ o b4 + n−1 b−1/2 .
2 π x (1 − x)
MSE f˜TS,j (x) =

2

c2

b4 +

MSE-optimal smoothing parameters are
bTS,j = c (1 − c ) λ (c )
∗

 2

2

(
∗

bJLN,j =

√ √

16 π

2/9

(
√ √

16 π

)2/9

f (x)

x (1 − x)p2j (x)n−2/9 ,

)2/9

f (x)

n−2/9 ,x (1 − x)q2j (x)which yield the optimal MSEs


8/9
o
f (x)
9
2/9
˜
n−8/9 ,
MSE fTS,j (x) ∼ 8/9 γ (c ) pj (x)√ √8
2 π x (1 − x)

8/9
n
o
f (x)
9 2/9
MSE ∗ f˜JLN,j (x) ∼ 8/9 qj (x)n−8/9 ,√ √8
2 π x (1 − x)
∗

n

where

(
γ (c ) =√1 + c 5/2 (1 + c )1/2 − 2 2c 3/2



)8/9

c 1/4 (1 + c )1/2 (1 − c )9/4.

Observe that the MSE-optimal smoothing parameters are O n−2/9 = O h∗2 , where h∗ is the MSE-optimal bandwidth for
fourth-order kernel estimators, or TS- or JLN-MBC
 estimators using symmetric second-order kernels. As a result, the optimal
MSEs of f˜TS,j (x) and f˜JLN,j (x) become O n−8/9 , as with MBC estimation using the second-order kernels. This convergencerate is faster than O n−4/5 , the optimal convergence rate in the MSEs of fˆB (x) and fˆMB (x) for interior x. On the other hand,for boundary x, the MSEs of f˜TS,j (x) and f˜JLN,j (x) are O b4 + n−1 b−1 . The MSE-optimal smoothing parameter bF = O n−1/5achieves the optimal MSE of O n−4/5, which is also faster than O n


−2/3



, the optimal convergence rate in the MSEs of fˆB (x)and fˆMB (x) for boundary x.
The undesirable convergence rate O n−1 b−1 over boundary regions does not affect the global properties of theseestimators. By the trimming argument in Chen (1999), mean integrated squared errors (MISEs) of the MBC estimators are

n

Z 1

b4

o

MISE f˜TS,j (x) =p2j (x) dx +n−1 b−1/2 λ (c )

√
2 π

Z 1
√

f (x)

dx + o b4 + n−1 b−1/2 ,x (1 − x)(1 − c ) 00Z 1o−1 −1/2 Z 1

n
b
f
x
(
)
q2j (x) dx +MISE f˜JLN,j (x) = b4
dx + o b4 + n−1 b−1/2 ,√√2 π
x (1 − x)
0
0

n

c2

2



478

M. Hirukawa / Computational Statistics and Data Analysis 54 (2010) 473–495

provided that all integrals are finite. MISE-optimal smoothing parameters are given byb∗∗
TS,j =

2

(R 1
0

∗∗

bJLN,j =

(R 1

2/9

c (1 − c ) λ (c )

 2

0

√

√

√ R1
16 π 0 p2j (x) dx)2/9√ R1 2n−2/9 .

f (x) / x (1 − x)dx

16 π

)2/9

f (x) / x (1 − x)dx

q (x) dx
0 jn−2/9 ,

Therefore, the optimal MISEs become

n

9

o

γ (c )
8/9

MISE ∗∗ f˜TS,j (x) ∼
MISE

∗∗

n

Z 1

8

p2j (x) dx

1/9 

0

1/9 Z 1
o
1
2
˜fJLN,j (x) ∼ 9
qj (x) dx
√
88/9

2 π

0

1

Z 1
√

√
2 π
Z 1

0

√

x ( 1 − x)f ( x)x (1 − x)

0

8/9

f (x)

dx

8/9
dx

n−8/9 ,

n−8/9 .

Furthermore, the multiplier γ (c ) in the optimal MISE for the TS-MBC estimator is minimized at c ∗ ≈ 0.2636; this optimalvalue is considered throughout.
2.3. Normalization
Neither f˜TS,j (x) nor f˜JLN,j (x) integrates to one. In general, MBC leads to lack of normalization, even if symmetric second-order kernels are employed; see Section 2.2 in JLN, for example. While lack of normalization in f˜TS,j (x) and f˜JLN,j (x) may beignored in some applications, Gouriéroux and Monfort (2006), for example, argue that this issue should be resolved for theestimation of recovery rate densities in credit risk modelling, and propose two renormalized beta kernel density estimators.
Taking the structures of f˜TS,j (x) and f˜JLN,j (x) into account, we adopt their ‘‘macro’’ approach to obtain the renormalized MBC
estimatorsRf˜TS,j (x) = R 10

f˜TS,j (x)f˜TS,j (x) dx

Rf˜JLN,j (x) = R 1
0,

f˜JLN,j (x)f˜JLN,j (x) dx

.

Since

Z 1
E

=1+

0

Z 1
E

b2

Z 1

c (1 − c )0f˜TS,j (x) dx

f˜JLN,j (x) dx

Z 1= 1 + b20

pj (x) dx + o b2 ,qj (x) dx + o b2 ,



0

R
˜R
provided that pj (x) and qj (x) are integrable, biases of f˜TS,j (x) and fJLN,j (x) can be approximated byonRbias f˜TS,j (x) ∼1

pj (x) −Z 1

pj (x) dx b2 ,c (1 − c )
0

Z 1
o 
R
bias f˜JLN
x
∼
q
x
−
q
x
dx
b2 .
(
)
(
)
(
)
j
j
,j

n

0

Their asymptotic variances are unaffected.
2.4. Further bias reduction
In principle, further bias reduction is possible after the regularity conditions are properly strengthened. Constructing a
multiplicative combination of (s + 1) different density estimators, we can generalize the TS-MBC estimator f˜TS,j (x) as(s)f˜TS,j (x) =s nYfˆj,b/cr (x)

oαs,r

,

r =0

where c0 = 1, c1 , . . . , cs ∈ (0, 1) are mutually different constants, and the exponent is

αs,r =(−1)s crs.
Q
cp − crs

p=0,p6=r

M. Hirukawa / Computational Statistics and Data Analysis 54 (2010) 473–495

479

Similarly, the sth iterated JLN-MBC estimator can be defined as
(s)
(s−1)
f˜JLN,j (x) = f˜JLN,j (x)(1 X K(j;x,b) (Xi )n)n i=1 f˜ (s−1) (Xi )

,

JLN,j

(0)

where f˜JLN,j (x) = fˆj,b (x). Provided that the true density f is 2 (s + 1) times continuously differentiable, for each of theseestimators, it can be shown that the order of magnitude in bias is O bs+1 while the variance remains O n−1 b−1/2 and



O n


−1 −1
b

n

(s)



o

for interior and boundary x, respectively. In particular, v ar f˜JLN,j (x) is first-order asymptotically equivalent to(0)−(4s+4)/(4s+5)
ˆ
v ar f˜JLN
and O n−(2s+2)/(2s+3) for interior and,j (x) = v ar fj,b (x) for interior x. Their optimal MSEs are O n
n

o

n

o

boundary x, respectively. Accordingly, as the number of iterations increases, global convergence rates of the iterated MBC
estimators when best implemented can be arbitrarily close to the parametric one. However, it is doubtful whether there ismuch gain in practice from these estimators, and thus we do not pursue this issue any further.
3. Finite sample performance
3.1. Smoothing parameter selection
Choosing the smoothing parameter b is an important practical issue. In the spirit of Jones and Henderson (2007b), we
propose a plug-in method with a beta density used as a reference. The plug-in smoothing parameters for f˜TS,MB (x) andf˜JLN,MB (x) (called ‘‘beta-referenced smoothing parameters’’ hereafter) are defined as the minimizers of asymptotic weighted
mean integrated squared errors (AWMISEs)nb̂TS.MB = arg min AWMISE f˜TS,MB (x)ob= arg min
b

b4

Z 1

c 2 (1 − c )20np̃2MB (x) w (x) dx +n−1 b−1/2 λ (c )

Z 1

√
2 π

√
0

g (x)

x ( 1 − x)w (x) dx,ob̂JLN,MB = arg min AWMISE f˜JLN,MB (x)b= arg min b4
bZ 1

q̃2MB (x) w (x) dx +0n−1 b−1/2

√
2 π

Z 1
√
0

g ( x)x (1 − x)w (x) dx,

where g (x) = xα−1 (1 − x)β−1 1 {0 ≤ x ≤ 1} /B (α, β) is the density function for the beta distribution with parameters(α, β), p̃MB (x) and q̃MB (x) can be obtained by replacing f (x) with g (x) in pMB (x) and qMB (x), respectively, and w (x) isa weighting function that ensures finiteness of integrals. Because of the expressions of p̃MB (x) and q̃MB (x), we choosew (x) = x5 (1 − x)5 . In practice, the parameters (α, β) are replaced by their method of moments estimates α̂, β̂ .
Analytical expressions of b̂TS,MB and b̂JLN,MB , as well as the beta-referenced smoothing parameter for fˆMB (x) in Jones andHenderson (2007b), denoted as b̂MB , are given in the Appendix.
We do not pursue the beta-referenced smoothing parameter for f˜TS,B (x) or f˜JLN,B (x); since extra terms are involved inpB (x) and qB (x), the minimizers of their AWMISEs take much more complicated forms than b̂TS,MB and b̂JLN,MB . From theviewpoint of practical relevance, these smoothing parameters are simply employed for f˜TS,B (x) and f˜JLN ,B (x) in the Monte
Carlo simulations below. Similarly, b̂MB is chosen as the smoothing parameter for fˆB (x). Strictly speaking, b̂TS,MB and b̂JLN,MB

are not AWMISE-optimal choices for f˜TS,B (x) and f˜JLN,B (x). However, when b̂TS,MB and b̂JLN,MB are chosen, optimal convergence
rates in MISEs for these estimators can be still achieved, and the Monte Carlo simulations indicate that they often outperformf˜TS,MB (x) and f˜JLN,MB (x).
3.2. Monte Carlo setup
Monte Carlo simulations compare the finite sample performance of the following 7 classes of estimators, or 14 estimators
in total: (i) bias-uncorrected estimator fˆj (x); (ii) ‘‘micro’’ estimator fˆjr (x) in Gouriéroux and Monfort (2006); (iii) ‘‘macro’’ (orrenormalized) estimator fˆjR (x) in Gouriéroux and Monfort (2006); (iv) TS-MBC estimator f˜TS,j (x); (v) renormalized TS-MBC
R˜
˜R
estimator f˜TS,j (x); (vi) JLN-MBC estimator fJLN,j (x); and (vii) renormalized JLN-MBC estimator fJLN,j (x), where ‘‘micro’’ and‘‘macro’’ estimators are defined as

K(j;x,b) (Xi )nfˆjr (x) =1Xn i =1R10

K(j;x,b) (Xi ) dx

,

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M. Hirukawa / Computational Statistics and Data Analysis 54 (2010) 473–495

Table 1
True distributions considered in Monte Carlo simulations.

1.
2.
3.
4.
5.
6.
7.
8.
9.
10.

Distribution

Density function f (x) , x ∈ [0, 1]

ShapeU [0, 1]
B (3, 1)
Truncated Exp (1/2)
Truncated N (0, 1/4)
B (3 , 3 )
B (1/2, 1/2)
(1/2) B (3, 1) + (1/2) B (1, 3)
B (2 , 4 )(1/2) B (5, 2) + (1/2) B (2, 5)(1/4) B (8, 2) + (3/4) B (2, 8)

1
3x2
2 exp (−2x) / {1 − exp (−2)}
2φ (2x) / {Φ (2) − 1/2}
30x2 (√
1 − x )2
1/ π  x (1 − x)(3/2) x2 + (1 − x)220x(1 − x)315 x4 (1 − x) + x (1 − x)4
18 x7 (1 − x) + 3x (1 − x)7

Horizontal
Increasing
Decreasing
Decreasing
Bell-shaped
U-shaped
U-shaped
Unimodal, right-skewed
Bimodal, symmetricBimodal, right-skewedfˆjR (x) = R 1

fˆj (x)0fˆj (x) dx,

respectively. The value of the constant c in each TS-MBC estimator is set equal to the MISE-optimal c ∗ = 0.2636. Tentrue distributions are considered, as listed in Table 1, and for each distribution, 1000 data sets of sample size n = 100 orn = 200 are simulated. For each simulated data set, the smoothing parameter is chosen via the following two methods: (i) asimple ‘‘rule-of-thumb’’ method (see Renault and Scaillet, 2004, and Gouriéroux and Monfort, 2006) such that b̂plain =σ̂ n−2/5 and b̂mbc = σ̂ n−2/9 for all bias-uncorrected and MBC estimators, respectively, where σ̂ is the sample standard
deviation; and (ii) the beta-referenced method described in the previous section, where b̂MB , b̂TS,MB and b̂JLN,MB are applied
for all bias-uncorrected, TS- and JLN-MBC estimators, respectively. The purposes of using two plug-in smoothing parameters
for a given density estimator are to speed up computation and to see how the beta-referenced method can improve theperformance of each estimator in comparison with the rule-of-thumb method. Since the beta-referenced method often
picks up a large value, estimated smoothing parameter values are trimmed at 1 so that their lengths may not exceed that of
the support. All density estimates are evaluated on an equally spaced grid of 999 points {0.001, 0.002, . . . , 0.999}. For eachestimator f¯ ,ISE f¯ =Z 1f¯ (x) − f (x)



0

2

dx ≈

1

999  Xf¯1000 j=1

j
1000




−f

j

2

1000

is computed over the grids, and then it is averaged over 1000 replications. Averages of integrated squared biases and standard
errors (defined as square roots of the estimates of asymptotic integrated variances) are also computed for reference.
3.3. Simulation results
Monte Carlo results, including descriptive statistics of smoothing parameters, are given in Tables 2–5. Observe that theresults are qualitatively similar across two sample sizes. Influences of trimming for smoothing parameters are limited in thesense that there are very few numbers of trimmed smoothing parameter estimates. Frequencies of trimming appear to belarge for Distribution 1 (=U [0, 1]). However, the bias-uncorrected and its renormalized estimators have no asymptotic
biases (and thus bias correction is unnecessary). For this distribution, simply setting b = O (1) (i.e. oversmoothing) is
optimal, and numbers of trimmed smoothing parameters are not an issue.
Comparing TS- and JLN-MBC estimators with their corresponding bias-uncorrected estimators, we find that JLN-MBC
estimators often improve the average ISE, whereas results are mixed for TS-MBC estimators. Even in the absence of
substantial improvement in the average ISE, f˜JLN,B (x) tends to reduce the average integrated squared bias from fˆB (x). In
addition, as suggested by the asymptotic results, TS-MBC estimators are less efficient than JLN-MBC estimators in thatstandard errors of the former are larger due to the extra constant λ (c ) ≥ 1. Superior performance of JLN-MBC over
TS-MBC is consistent with the simulation results reported in Jones and Signorini (1997).
However, the results do not necessarily support superiority of the modified beta kernel over the beta kernel. In addition,
the effect of normalization appears to be marginal in many cases. This is due to the fact that renormalization induces anextra bias term. Since the sign of this term depends on the true density, it may or may not lead to a less biased estimate.
Comparing two smoothing parameter choice methods for a given estimator, we also find that for JLN-MBC estimators, thebeta-referenced method works better for Distributions 1, 2, 3, 4, 6, and 7. For other distributions, the rule-of-thumb method
tends to perform better, but there is not much gain in the average ISE unless the true density is bimodal (e.g. Distributions
9 and 10). In short, as far as the shape of the true density does not deviate considerably from that of a beta distribution, thebeta-referenced method works well for JLN-MBC estimators. In contrast, the rule-of-thumb method tends to perform better
for TS-MBC estimators. This finding is revisited shortly, in relation to poor performance of f˜TS,MB (x). The results are mixed
for bias-uncorrected estimators.
Fig. 1 is prepared to visualize how two classes of MBC techniques work. Each panel displays average plots of density
estimates by bias-uncorrected beta (fˆB (x), denoted ‘‘Plain’’ in the legend), beta TS-MBC (fˆTS,B (x)), and beta JLN-MBC

M. Hirukawa / Computational Statistics and Data Analysis 54 (2010) 473–495

481

Table 2Average ISE computed on 1000 replications (n = 100; Distributions 1–5).
True distribution
1

2

3

4

5

ROT

BR

ROT

BR

ROT

BR

ROT

BR

ROT

BR

0.0283(0.0000)[0.2006]
0.0308(0.0000)[0.2004]0.0442(0.0085)[0.2073]
0.0322(0.0031)[0.1986]
0.0282(0.0000)[0.2007]0.0308(0.0000)[0.2006]

0.0072(0.0000)[0.1217]0.0057(0.0000)[0.1217]0.0187(0.0072)[0.1248]0.0068(0.0002)[0.1222]0.0068(0.0000)[0.1217]0.0056(0.0000)[0.1217]0.0382(0.0030)[0.2316]0.0410(0.0009)[0.2342]0.0659(0.0195)[0.2417]
0.0469(0.0095)[0.2305]
0.0384(0.0017)[0.2333]
0.0395(0.0010)[0.2331]0.0408(0.0202)[0.1766]
0.0324(0.0093)[0.1796]0.0484(0.0224)[0.1871]
0.0393(0.0197)[0.1776]
0.0359(0.0137)[0.1792]
0.0317(0.0105)[0.1786]

0.0321(0.0027)[0.2103]
0.0335(0.0010)[0.2122]0.0516(0.0135)[0.2195]
0.0384(0.0077)[0.2093]0.0323(0.0021)[0.2116]0.0326(0.0011)[0.2115]

0.0416(0.0256)[0.1456]
0.0451(0.0207)[0.1472]
0.0369(0.0129)[0.1533]
0.0368(0.0191)[0.1473]
0.0394(0.0219)[0.1475]0.0446(0.0212)[0.1470]0.0298(0.0009)[0.2073]
0.0310(0.0001)[0.2069]
0.0472(0.0118)[0.2137]
0.0332(0.0038)[0.2051]
0.0297(0.0009)[0.2073]
0.0309(0.0001)[0.2071]

0.0382(0.0281)[0.1299]0.0494(0.0337)[0.1297]
0.0396(0.0260)[0.1333]
0.0436(0.0289)[0.1302]
0.0383(0.0284)[0.1297]0.0494(0.0337)[0.1297]

0.0319(0.0073)[0.1961]
0.0255(0.0033)[0.1892]
0.0328(0.0094)[0.1935]
0.0239(0.0009)[0.1915]0.0324(0.0091)[0.1933]
0.0246(0.0012)[0.1918]

0.0338(0.0112)[0.1865]0.0249(0.0054)[0.1787]
0.0362(0.0144)[0.1834]
0.0213(0.0015)[0.1813]0.0352(0.0137)[0.1832]
0.0229(0.0022)[0.1816]0.0218(0.0000)[0.1852]
0.0250(0.0000)[0.1851]
0.0216(0.0000)[0.1852]
0.0248(0.0001)[0.1851]

0.0087(0.0000)[0.1363]
0.0068(0.0001)[0.1366]
0.0083(0.0000)[0.1363]
0.0066(0.0000)[0.1363]

0.0287(0.0016)[0.2153]
0.0428(0.0097)[0.2209]
0.0288(0.0014)[0.2156]
0.0304(0.0031)[0.2155]

0.0325(0.0060)[0.1994]
0.0420(0.0112)[0.2054]0.0306(0.0048)[0.2002]0.0305(0.0061)[0.1998]

0.0250(0.0023)[0.1943]0.0335(0.0069)[0.1981]0.0249(0.0018)[0.1953]
0.0274(0.0043)[0.1951]

0.0283(0.0097)[0.1654]0.0369(0.0057)[0.1689]0.0278(0.0082)[0.1666]
0.0323(0.0057)[0.1663]

0.0230(0.0007)[0.1922]0.0266(0.0017)[0.1925]
0.0226(0.0008)[0.1915]0.0244(0.0012)[0.1915]0.0306(0.0184)[0.1414]0.0437(0.0210)[0.1423]
0.0314(0.0192)[0.1406]
0.0436(0.0226)[0.1409]

0.0315(0.0126)[0.1850]0.0206(0.0042)[0.1749]
0.0326(0.0157)[0.1800]
0.0192(0.0021)[0.1779]

0.0428(0.0064)[0.2050]
0.0332(0.0022)[0.1981]0.0439(0.0079)[0.2011]0.0310(0.0014)[0.1997]

0.0269(0.0001)[0.1630]
0.0257(0.0001)[0.1630]
0.0272(0.0000)[0.1635]
0.0260(0.0001)[0.1634]

0.0144(0.0000)[0.1269]
0.0116(0.0000)[0.1270]0.0144(0.0000)[0.1271]
0.0117(0.0000)[0.1272]

0.0330(0.0005)[0.1897]
0.0320(0.0017)[0.1892]
0.0341(0.0001)[0.1915]0.0326(0.0017)[0.1905]

0.0234(0.0043)[0.1560]
0.0238(0.0084)[0.1552]
0.0215(0.0004)[0.1592]
0.0223(0.0063)[0.1576]

0.0284(0.0007)[0.1718]
0.0274(0.0018)[0.1717]0.0289(0.0002)[0.1731]0.0276(0.0017)[0.1723]0.0215(0.0036)[0.1432]
0.0227(0.0059)[0.1429]0.0212(0.0018)[0.1450]
0.0220(0.0048)[0.1441]0.0270(0.0001)[0.1686]
0.0258(0.0002)[0.1682]
0.0274(0.0001)[0.1692]
0.0262(0.0002)[0.1689]0.0167(0.0041)[0.1296]0.0168(0.0057)[0.1291]
0.0167(0.0035)[0.1303]
0.0159(0.0043)[0.1304]

0.0233(0.0028)[0.1561]
0.0228(0.0037)[0.1527]
0.0235(0.0023)[0.1567]
0.0235(0.0021)[0.1559]

0.0270(0.0050)[0.1502]0.0268(0.0066)[0.1465]0.0276(0.0045)[0.1505]
0.0258(0.0036)[0.1500]

0.04560.002100.4434
0.496060

0.03040.002200.08910.028200.04140.002600.20490.175830.03950.002400.30900.202012

0.02990.001700.03830.006400.10340.004700.38800.16615

0.06900.005000.10340.056200.09390.005900.20070.116920.08960.005500.35490.161930.06770.003900.05370.033700.10340.004700.30300.11543

0.06900.005000.14610.0274
00.09390.005900.20190.07231

0.08960.005500.27240.08471

0.06770.003900.08480.02560Density estimator:
fˆB

fˆMB

fˆBr
r
fˆMB

fˆBR
R
fˆMB

f˜TS,Bf˜TS,MB
R
f˜TS
,B

R
f˜TS
,MB

f˜JLN,Bf˜JLN,MB
R
f˜JLN
,B

R
f˜JLN
,MB

Smoothing parameter:
Plain:
Mean
Std. Dev.
#{Trimmed}
TS-MBC:
Mean
Std. Dev.
#{Trimmed}
JLN-MBC:
Mean
Std. Dev.
#{Trimmed}

Note: ‘‘ROT’’ and ‘‘BR’’ in column headings denote ‘‘rule-of-thumb’’ and ‘‘beta-referenced’’ smoothing parameter choice methods. Numbers in parentheses
and brackets for density estimators are averages of integrated squared biases and standard errors (defined as square roots of the estimates of asymptotic
integrated variances). ‘‘Mean’’, ‘‘Std. Dev.’’, and ‘‘#{Trimmed}’’ for smoothing parameters are averages, standard deviations, and numbers of smoothing
parameters trimmed at one.

(fˆJLN,B (x)) estimators for n = 100, where average is taken over 1000 replications for each grid. For comparison, resultsfrom two smoothing parameters are provided for a given distribution. We can see that if the bias-uncorrected estimator

482

M. Hirukawa / Computational Statistics and Data Analysis 54 (2010) 473–495

Table 3
Average ISE computed on 1000 replications (n = 100; Distributions 6–10).
True distribution
6

7

8

9

10

ROT

BR

ROT

BR

ROT

BR

ROT

BR

ROT

BR

0.2801(0.2524)[0.2015]0.2850(0.2496)[0.2137]
0.1382(0.0700)[0.2370]0.2422(0.2009)[0.2177]0.2550(0.2194)[0.2139]0.2842(0.2483)[0.2149]

0.3090(0.2837)[0.1843]0.3184(0.2837)[0.1948]0.1533(0.0893)[0.2174]0.2708(0.2325)[0.1996]0.2815(0.2482)[0.1966]
0.3169(0.2817)[0.1966]

0.0310(0.0023)[0.2029]0.0337(0.0014)[0.2060]
0.0506(0.0112)[0.2136]0.0377(0.0071)[0.2028]0.0316(0.0015)[0.2051]
0.0326(0.0014)[0.2047]

0.0255(0.0104)[0.1502]0.0271(0.0100)[0.1528]0.0291(0.0051)[0.1599]
0.0236(0.0097)[0.1527]0.0243(0.0079)[0.1533]
0.0267(0.0102)[0.1524]0.0384(0.0102)[0.2106]
0.0331(0.0063)[0.2044]0.0430(0.0148)[0.2092]
0.0315(0.0049)[0.2051]
0.0385(0.0115)[0.2083]
0.0327(0.0048)[0.2062]0.0449(0.0210)[0.1906]
0.0379(0.0155)[0.1844]0.0568(0.0315)[0.1891]
0.0356(0.0138)[0.1848]0.0461(0.0234)[0.1877]
0.0366(0.0133)[0.1861]

0.0387(0.0122)[0.1977]
0.0356(0.0101)[0.1920]0.0453(0.0183)[0.1968]
0.0360(0.0114)[0.1915]0.0375(0.0124)[0.1951]
0.0359(0.0097)[0.1931]0.0542(0.0446)[0.1398]
0.0604(0.0510)[0.1379]
0.0686(0.0586)[0.1391]
0.0637(0.0522)[0.1369]
0.0527(0.0442)[0.1371]0.0602(0.0510)[0.1372]

0.1004(0.0728)[0.2089]
0.1149(0.0861)[0.2068]
0.1140(0.0837)[0.2110]0.1222(0.0957)[0.2029]
0.0969(0.0699)[0.2080]0.1086(0.0809)[0.2051]0.1336(0.1147)[0.1717]0.1657(0.1448)[0.1738]
0.1545(0.1341)[0.1758]
0.1637(0.1467)[0.1691]0.1358(0.1165)[0.1723]0.1505(0.1315)[0.1702]0.3222(0.3015)[0.1816]
0.3412(0.3135)[0.1929]0.2919(0.2646)[0.1944]
0.3399(0.3129)[0.1933]

0.2971(0.2687)[0.1947]0.3252(0.2815)[0.2083]
0.2722(0.2354)[0.2072]
0.3220(0.2822)[0.2070]0.0257(0.0035)[0.1863]0.0304(0.0038)[0.1909]
0.0257(0.0022)[0.1890]0.0276(0.0032)[0.1881]0.0255(0.0069)[0.1658]
0.0301(0.0067)[0.1693]0.0248(0.0047)[0.1690]0.0284(0.0069)[0.1679]

0.0411(0.0189)[0.1989]
0.0363(0.0147)[0.1918]0.0406(0.0206)[0.1942]0.0361(0.0143)[0.1923]0.0496(0.0070)[0.2284]0.0456(0.0046)[0.2228]0.0493(0.0076)[0.2252]0.0456(0.0045)[0.2232]

0.0448(0.0239)[0.1859]
0.0491(0.0276)[0.1812]0.0431(0.0239)[0.1825]
0.0492(0.0276)[0.1814]0.0622(0.0557)[0.1367]
0.0663(0.0622)[0.1363]
0.0599(0.0544)[0.1336]
0.0651(0.0614)[0.1342]0.1342(0.1125)[0.1970]
0.1969(0.1722)[0.2013]0.1247(0.1044)[0.1949]
0.1431(0.1240)[0.1923]0.1402(0.1175)[0.1931]0.2060(0.1778)[0.1979]
0.1318(0.1104)[0.1913]0.1505(0.1288)[0.1888]0.2787(0.2519)[0.1648]0.3182(0.2948)[0.1663]0.2555(0.2210)[0.1748]0.3103(0.2830)[0.1724]

0.2617(0.2280)[0.1739]0.3007(0.2693)[0.1758]
0.2423(0.1996)[0.1838]
0.2952(0.2590)[0.1817]0.0291(0.0014)[0.1659]
0.0270(0.0022)[0.1661]
0.0298(0.0006)[0.1678]0.0272(0.0021)[0.1667]

0.0262(0.0029)[0.1495]
0.0255(0.0043)[0.1496]0.0266(0.0017)[0.1517]0.0256(0.0041)[0.1504]

0.0316(0.0070)[0.1693]0.0346(0.0105)[0.1664]0.0319(0.0067)[0.1696]0.0351(0.0091)[0.1689]0.0373(0.0136)[0.1588]0.0416(0.0185)[0.1558]
0.0376(0.0133)[0.1590]0.0409(0.0158)[0.1586]

0.0383(0.0137)[0.1603]
0.0433(0.0202)[0.1585]0.0380(0.0139)[0.1593]0.0436(0.0200)[0.1589]

0.0512(0.0383)[0.1312]0.0615(0.0511)[0.1308]0.0505(0.0383)[0.1296]0.0613(0.0511)[0.1301]

0.1011(0.0761)[0.1706]
0.1123(0.0902)[0.1692]
0.1089(0.0825)[0.1722]0.1170(0.0941)[0.1703]0.1163(0.0951)[0.1601]
0.1324(0.1121)[0.1593]0.1265(0.1035)[0.1622]0.1357(0.1147)[0.1600]

0.05590.002100.07710.015900.05020.002000.18020.14207

0.02820.001900.04590.012400.04240.001900.23720.1892100.04530.002700.10350.025600.12680.004800.10480.019900.11370.004500.18920.0852
1

0.06390.004200.04330.032300.09610.004300.37940.116930.10260.0061
00.11330.020800.12680.004800.10840.013300.11370.004500.17740.0606
00.06390.004200.08790.020000.09610.004300.24400.062300.10260.006100.13550.01990Density estimator:
fˆB

fˆMB

fˆBr
r
fˆMB

fˆBR
R
fˆMB

f˜TS,Bf˜TS,MB
R
f˜TS
,B

R
f˜TS
,MB

f˜JLN,Bf˜JLN,MB
R
f˜JLN
,B

R
f˜JLN
,MB

Smoothing parameter:
Plain:
Mean
Std. Dev.
#{Trimmed}
TS-MBC:
Mean
Std. Dev.
#{Trimmed}
JLN-MBC:
Mean
Std. Dev.
#{Trimmed}

Note: ‘‘ROT’’ and ‘‘BR’’ in column headings denote ‘‘rule-of-thumb’’ and ’’beta-referenced’’ smoothing parameter choice methods. Numbers in parentheses
and brackets for density estimators are averages of integrated squared biases and standard errors (defined as square roots of the estimates of asymptotic
integrated variances). ‘‘Mean’’, ‘‘Std. Dev.’’, and ‘‘#{Trimmed}’’ for smoothing parameters are averages, standard deviations, and numbers of smoothing
parameters trimmed at one.

underestimates (overestimates) the density, JLN-MBC corrects the estimate in an upward (downward) direction, as reported
in Hirukawa (2006). Such a bias correction mechanism is not obvious for TS-MBC.

M. Hirukawa / Computational Statistics and Data Analysis 54 (2010) 473–495

483

Table 4
Average ISE computed on 1000 replications (n = 200; Distributions 1–5).
True distribution
1

2

3

4

5

ROT

BR

ROT

BR

ROT

BR

ROT

BR

ROT

BR

0.0172(0.0000)[0.1520]0.0188(0.0000)[0.1519]0.0283(0.0070)[0.1567]0.0204(0.0026)[0.1504]
0.0171(0.0000)[0.1521]0.0188(0.0000)[0.1520]0.0036(0.0000)[0.0860]
0.0029(0.0000)[0.0860]
0.0135(0.0072)[0.0882]
0.0038(0.0002)[0.0864]0.0034(0.0000)[0.0860]
0.0029(0.0000)[0.0860]

0.0231(0.0020)[0.1757]
0.0241(0.0003)[0.1775]
0.0461(0.0191)[0.1828]0.0293(0.0070)[0.1748]0.0228(0.0011)[0.1768]
0.0234(0.0004)[0.1768]

0.0257(0.0146)[0.1323]0.0173(0.0053)[0.1345]0.0389(0.0252)[0.1399]0.0276(0.0168)[0.1327]0.0215(0.0097)[0.1340]0.0173(0.0061)[0.1337]0.0198(0.0019)[0.1596]0.0206(0.0006)[0.1608]0.0349(0.0119)[0.1660]
0.0255(0.0066)[0.1586]
0.0198(0.0015)[0.1604]0.0202(0.0007)[0.1604]

0.0270(0.0176)[0.1109]0.0246(0.0113)[0.1122]0.0261(0.0132)[0.1168]
0.0227(0.0137)[0.1118]
0.0249(0.0148)[0.1122]0.0244(0.0117)[0.1119]0.0182(0.0006)[0.1569]
0.0191(0.0001)[0.1566]
0.0305(0.0092)[0.1613]0.0214(0.0033)[0.1552]
0.0181(0.0006)[0.1569]
0.0190(0.0001)[0.1568]

0.0296(0.0245)[0.0942]0.0347(0.0260)[0.0941]0.0323(0.0254)[0.0969]
0.0299(0.0222)[0.0945]0.0299(0.0248)[0.0940]0.0347(0.0260)[0.0941]0.0192(0.0044)[0.1474]
0.0156(0.0020)[0.1433]
0.0199(0.0056)[0.1459]0.0147(0.0005)[0.1447]0.0197(0.0055)[0.1458]
0.0149(0.0007)[0.1448]0.0202(0.0069)[0.1397]
0.0150(0.0032)[0.1349]
0.0217(0.0089)[0.1378]0.0129(0.0008)[0.1365]
0.0214(0.0086)[0.1378]
0.0135(0.0011)[0.1367]

0.0123(0.0000)[0.1361]0.0142(0.0000)[0.1360]
0.0122(0.0000)[0.1361]0.0141(0.0000)[0.1360]

0.0045(0.0000)[0.0968]
0.0035(0.0000)[0.0969]0.0042(0.0000)[0.0968]
0.0034(0.0000)[0.0968]

0.0161(0.0012)[0.1580]0.0245(0.0065)[0.1617]0.0160(0.0010)[0.1583]
0.0174(0.0020)[0.1584]

0.0174(0.0040)[0.1458]
0.0242(0.0102)[0.1502]
0.0162(0.0032)[0.1464]0.0153(0.0040)[0.1462]0.0145(0.0017)[0.1429]0.0200(0.0051)[0.1453]0.0143(0.0014)[0.1435]
0.0165(0.0032)[0.1435]

0.0168(0.0067)[0.1229]0.0217(0.0063)[0.1256]
0.0161(0.0055)[0.1238]
0.0181(0.0049)[0.1236]0.0130(0.0005)[0.1410]0.0150(0.0010)[0.1410]0.0128(0.0006)[0.1406]
0.0141(0.0008)[0.1406]

0.0242(0.0172)[0.1010]0.0344(0.0183)[0.1015]0.0250(0.0180)[0.1004]
0.0348(0.0197)[0.1006]

0.0194(0.0088)[0.1347]
0.0117(0.0024)[0.1283]0.0208(0.0111)[0.1316]0.0107(0.0012)[0.1301]

0.0249(0.0051)[0.1462]0.0178(0.0013)[0.1415]
0.0263(0.0063)[0.1437]
0.0162(0.0007)[0.1426]

0.0151(0.0000)[0.1199]
0.0147(0.0001)[0.1199]
0.0152(0.0000)[0.1202]0.0148(0.0000)[0.1201]

0.0073(0.0000)[0.0899]
0.0060(0.0000)[0.0899]
0.0072(0.0000)[0.0900]
0.0060(0.0000)[0.0900]

0.0182(0.0003)[0.1395]
0.0178(0.0008)[0.1392]
0.0185(0.0000)[0.1405]
0.0180(0.0009)[0.1399]

0.0131(0.0032)[0.1137]0.0139(0.0060)[0.1132]
0.0110(0.0003)[0.1156]
0.0128(0.0048)[0.1146]

0.0161(0.0005)[0.1264]0.0160(0.0013)[0.1264]0.0162(0.0002)[0.1271]
0.0160(0.0012)[0.1267]

0.0125(0.0024)[0.1059]0.0141(0.0045)[0.1058]
0.0120(0.0013)[0.1070]0.0138(0.0041)[0.1064]0.0152(0.0001)[0.1239]0.0148(0.0001)[0.1236]0.0153(0.0000)[0.1241]0.0149(0.0001)[0.1240]0.0096(0.0034)[0.0933]
0.0094(0.0040)[0.0930]
0.0094(0.0029)[0.0937]
0.0085(0.0030)[0.0938]

0.0133(0.0017)[0.1144]0.0130(0.0020)[0.1124]0.0132(0.0013)[0.1148]0.0132(0.0012)[0.1142]0.0148(0.0031)[0.1097]
0.0140(0.0038)[0.1074]
0.0149(0.0027)[0.1100]
0.0130(0.0022)[0.1095]

0.03460.001100.44480.394662

0.02320.001200.06990.014600.03150.001400.14390.09502

0.03000.001300.26370.13407

0.02270.000900.02910.003400.08870.002900.38050.162410

0.05950.003100.08590.032400.08070.0036
00.15400.062500.07700.003400.33260.133400.05820.002400.04810.028500.08870.002900.30230.11372

0.05950.003100.13030.017900.08070.003600.16570.046700.07700.003400.25030.063500.05820.002400.07160.01450Density estimator:
fˆB

fˆMB

fˆBr
r
fˆMB

fˆBR
R
fˆMB

f˜TS,Bf˜TS,MB
R
f˜TS
,B

R
f˜TS
,MB

f˜JLN,Bf˜JLN,MB
R
f˜JLN
,B

R
f˜JLN
,MB

Smoothing parameter:
Plain:
Mean
Std. Dev.
#{Trimmed}
TS-MBC:
Mean
Std. Dev.
#{Trimmed}
JLN-MBC:
Mean
Std. Dev.
#{Trimmed}

Note: ‘‘ROT’’ and ‘‘BR’’ in column headings denote ‘‘rule-of-thumb’’ and ‘‘beta-referenced’’ smoothing parameter choice methods. Numbers in parentheses
and brackets for density estimators are averages of integrated squared biases and standard errors (defined as square roots of the estimates of asymptotic
integrated variances). ‘‘Mean’’, ‘‘Std. Dev.’’, and ‘‘#{Trimmed}’’ for smoothing parameters are averages, standard deviations, and numbers of smoothing
parameters trimmed at one.

Tables 2–5 also indicate that either the modified beta TS-MBC estimator f˜TS,MB (x) or the ‘‘micro’’ beta estimator fˆBr (x)consistently performs inferiorly. The reasons for their poor performance can be explained as follows. First, when it comesto a large bias in f˜TS,MB (x), a TS-MBC estimator depends on two smoothing parameters b and b/c. Controlling both b and b/c

484

M. Hirukawa / Computational Statistics and Data Analysis 54 (2010) 473–495

Table 5
Average ISE computed on 1000 replications (n = 200; Distributions 6–10).
True distribution
6

7

8

9

10

ROT

BR

ROT

BR

ROT

BR

ROT

BR

ROT

BR

0.2356(0.2186)[0.1552]0.2355(0.2133)[0.1650]
0.0946(0.0539)[0.1821]0.1918(0.1668)[0.1675]0.2106(0.1891)[0.1640]0.2350(0.2127)[0.1655]

0.2662(0.2507)[0.1421]0.2691(0.2476)[0.1508]
0.1068(0.0694)[0.1674]
0.2219(0.1986)[0.1537]0.2383(0.2183)[0.1509]0.2681(0.2466)[0.1516]0.0184(0.0015)[0.1544]0.0198(0.0007)[0.1565]
0.0337(0.0112)[0.1619]0.0237(0.0057)[0.1540]0.0185(0.0010)[0.1558]
0.0192(0.0007)[0.1557]

0.0168(0.0080)[0.1148]0.0172(0.0068)[0.1169]0.0206(0.0079)[0.1222]0.0178(0.0097)[0.1162]0.0155(0.0060)[0.1169]
0.0169(0.0070)[0.1164]

0.0237(0.0064)[0.1585]0.0198(0.0033)[0.1544]0.0262(0.0091)[0.1576]0.0187(0.0023)[0.1550]
0.0239(0.0073)[0.1571]0.0194(0.0024)[0.1556]

0.0280(0.0141)[0.1431]0.0219(0.0090)[0.1386]
0.0353(0.0208)[0.1420]0.0196(0.0073)[0.1390]0.0291(0.0157)[0.1413]0.0207(0.0073)[0.1399]0.0247(0.0084)[0.1486]
0.0219(0.0061)[0.1444]0.0287(0.0123)[0.1480]0.0221(0.0067)[0.1443]0.0242(0.0086)[0.1470]0.0219(0.0058)[0.1453]0.0457(0.0393)[0.1064]0.0514(0.0446)[0.1047]
0.0619(0.0548)[0.1060]
0.0551(0.0465)[0.1039]0.0448(0.0390)[0.1044]0.0514(0.0447)[0.1043]0.0794(0.0629)[0.1574]
0.0884(0.0714)[0.1548]0.0862(0.0686)[0.1584]
0.0968(0.0810)[0.1526]
0.0752(0.0591)[0.1566]
0.0857(0.0690)[0.1543]

0.1077(0.0969)[0.1307]0.1329(0.1214)[0.1313]0.1263(0.1152)[0.1331]0.1381(0.1278)[0.1278]
0.1073(0.0966)[0.1306]
0.1209(0.1101)[0.1290]0.2916(0.2799)[0.1348]0.3087(0.2927)[0.1440]0.2607(0.2454)[0.1438]
0.3081(0.2929)[0.1435]

0.2639(0.2478)[0.1451]
0.2867(0.2617)[0.1558]
0.2376(0.2169)[0.1540]0.2847(0.2621)[0.1543]0.0146(0.0026)[0.1374]
0.0174(0.0029)[0.1409]0.0142(0.0017)[0.1391]0.0152(0.0020)[0.1387]

0.0158(0.0057)[0.1228]0.0185(0.0052)[0.1255]
0.0147(0.0039)[0.1249]
0.0173(0.0052)[0.1242]0.0269(0.0144)[0.1451]
0.0231(0.0110)[0.1400]
0.0270(0.0156)[0.1421]0.0228(0.0105)[0.1406]

0.0276(0.0057)[0.1635]
0.0250(0.0036)[0.1593]0.0277(0.0061)[0.1614]0.0249(0.0034)[0.1598]

0.0320(0.0199)[0.1359]0.0349(0.0226)[0.1321]0.0311(0.0199)[0.1336]0.0349(0.0224)[0.1326]

0.0563(0.0523)[0.1017]0.0622(0.0592)[0.1012]0.0545(0.0510)[0.0995]
0.0614(0.0586)[0.0998]

0.1154(0.1033)[0.1447]
0.1649(0.1512)[0.1469]
0.1041(0.0929)[0.1428]0.1206(0.1098)[0.1410]0.1195(0.1071)[0.1424]0.1730(0.1583)[0.1450]
0.1089(0.0975)[0.1407]0.1263(0.1144)[0.1389]0.2455(0.2305)[0.1225]0.2857(0.2724)[0.1237]
0.2221(0.2031)[0.1293]0.2779(0.2626)[0.1279]0.2268(0.2079)[0.1294]0.2658(0.2481)[0.1309]
0.2064(0.1829)[0.1362]0.2597(0.2396)[0.1349]

0.0159(0.0009)[0.1224]0.0151(0.0014)[0.1225]
0.0160(0.0005)[0.1235]
0.0151(0.0013)[0.1228]0.0147(0.0022)[0.1106]
0.0148(0.0032)[0.1107]0.0145(0.0013)[0.1119]0.0148(0.0031)[0.1110]0.0186(0.0048)[0.1241]0.0208(0.0072)[0.1222]0.0187(0.0047)[0.1242]0.0209(0.0064)[0.1237]

0.0227(0.0099)[0.1160]
0.0260(0.0136)[0.1140]0.0228(0.0098)[0.1161]
0.0251(0.0119)[0.1157]

0.0246(0.0104)[0.1173]
0.0290(0.0155)[0.1158]
0.0245(0.0107)[0.1165]0.0290(0.0153)[0.1162]0.0413(0.0334)[0.0969]
0.0532(0.0465)[0.0965]
0.0410(0.0336)[0.0957]
0.0531(0.0465)[0.0960]

0.0811(0.0671)[0.1251]
0.0927(0.0800)[0.1238]0.0855(0.0710)[0.1258]
0.0971(0.0839)[0.1246]

0.0944(0.0828)[0.1179]
0.1089(0.0978)[0.1171]
0.1009(0.0886)[0.1190]0.1117(0.1002)[0.1176]

0.04240.001100.05720.007200.03800.001000.12230.049900.02130.001000.03430.005800.03210.001000.1591
0.0983
30.03440.001400.07610.011600.10880.002800.08750.010100.09750.002700.15240.040000.05470.002500.03650.020000.08230.002600.30150.078910.08820.0037
00.09480.010900.10880.002800.09190.007100.09750.002700.14610.030300.05470.002500.07490.011500.08230.002600.20030.039500.08820.003700.11420.01070Density estimator:
fˆB

fˆMB

fˆBr
r
fˆMB

fˆBR
R
fˆMB

f˜TS,Bf˜TS,MB
R
f˜TS
,B

R
f˜TS
,MB

f˜JLN,Bf˜JLN,MB
R
f˜JLN
,B

R
f˜JLN
,MB

Smoothing parameter:
Plain:
Mean
Std. Dev.
#{Trimmed}
TS-MBC:
Mean
Std. Dev.
#{Trimmed}
JLN-MBC:
Mean
Std. Dev.
#{Trimmed}

Note: ‘‘ROT’’ and ‘‘BR’’ in column headings denote ‘‘rule-of-thumb’’ and ‘‘beta-referenced’’ smoothing parameter choice methods. Numbers in parentheses
and brackets for density estimators are averages of integrated squared biases and standard errors (defined as square roots of the estimates of asymptotic
integrated variances). ‘‘Mean’’, ‘‘Std. Dev.’’, and ‘‘#{Trimmed}’’ for smoothing parameters are averages, standard deviations, and numbers of smoothing
parameters trimmed at one.

is a cumbersome task. Because 0 < c < 1, the density estimator using b/c tends to be oversmoothed, which is potentiallya source of a large bias in every TS-MBC estimator. On the other hand, if we make b too short in order to have a reasonable
length of b/c, additional variability is introduced to the other estimator using b due to undersmoothing. In addition, whenM. Hirukawa / Computational Statistics and Data Analysis 54 (2010) 473–495

485

the modified beta kernel is employed, these two smoothing parameters also play a role of determining the boundary region
explicitly (e.g. [0, 2b)∪(1 − 2b, 1] for the density estimator using b). Unless b is short enough, there is a very narrow ‘interior’
region for the density estimator using b/c. This aspect is also thought to amplify the bias in f˜TS,MB (x). In other words, when
TS-MBC estimation is applied, it is desirable to use a smoothing parameter choice method that tends to pick up a smallvalue consistently. This explains why the rule-of-thumb method in general performs better in TS-MBC estimation than thebeta-referenced method.
Second, Fig. 2 exhibits average plots of selected density estimates based on fˆBr (x). For brevity, only plots based on therule-of-thumb method are provided. We can see from Panels (a), (b) and (d) that the density estimates tend to be upward
biased substantially near the boundary, wherever the tail of the true density is thick; on the other hand, this issue is notserious if the tail is thin (e.g. left tail of Panel (a)). In fact, fˆBr (x) possesses two kinds of bias terms. Other than the same

bias term as fˆB (x) has = (1 − 2x) f 0 (x) + (1/2) x (1 − x) f 00 (x) b , the estimator generates an extra O (b) bias that is
induced by renormalization and thus peculiar to this estimator. The extra bias comes from the fact that the normalization
constant does not become unity near the boundary. More precisely, if the data point Xi lies in an O (b) boundary region,
R1√ 
KB(x/b+1,(1−x)/b+1) (Xi ) dx ∼ Φκ ∈ (1/2, 1), where Φ (·) is the standard normal distribution function; the derivation0
is similar to the proof for the variance part of Theorem 2, and thus it is omitted. In finite samples, the O (b) boundary region
is not negligible, and a number of observations are likely to be concentrated near the boundary when the true density has a
R1
thick tail. Then, the ‘renormalized’ contributions KB(x/b+1,(1−x)/b+1) (Xi ) / 0 KB(x/b+1,(1−x)/b+1) (Xi ) dx from these observations
tend to be inflated. As a result, at a design point x toward the boundary, the inflated contributions dominate due to proximityof x to the observations in the boundary region, leading to an upward biased estimate. A similar argument can also explain
superior performance of fˆBr (x) for Distribution 6 in Panel (c). Moreover, as mentioned above, the bias-uncorrected and

its renormalized estimators have no asymptotic biases for Distribution 1. Nonetheless, in this case, fˆBr (x) still generates
a substantially large bias; indeed, the extra bias induced by renormalization does not disappear even in this case.

4. An application: Estimating the density of eruption durations of the Old Faithful Geyser
We apply two MBC techniques to a data set of eruption durations of the Old Faithful Geyser in Yellow Stone National Park,
Wyoming, USA. The data set is famous and applied frequently in the literature. Many textbooks on nonparametric statistics(e.g. Silverman, 1986; Loader, 1999) adopt it as an example of density estimation. Azzalini and Bowman (1990) also analyze
the data set extensively. The data set can be found in many sources. The one used in this section has been downloaded fromthe web site for Wasserman (2006).
The data set contains durations of 272 consecutive eruptions, ranging from 1.6 to 5.1 minutes. As in Gu (2002, p. 191), thedensity of eruption durations is assumed to have a compact support [1.5, 5.25]. An extension of beta density estimation to
any compact support is easy. In this example, an original observation Xi ∈ [1.5, 5.25] can be mapped onto the unit interval by
the transformation Zi = (Xi − 1.5) /3.75. Then, beta density estimation can be implemented using the transformed data Zi .
It is widely believed that the density is bimodal, which would be the closest to Distribution 10 in our Monte Carlosimulations. Tables 3 and 5 indicate that for this distribution, the beta kernel and the rule-of-thumb method perform better
than the modified beta kernel and the beta-referenced method, respectively. Renormalization does not necessarily decrease
the average ISE. Therefore, the following three density estimators are considered: (i) bias-uncorrected beta estimatorfˆB (x); (ii) beta TS-MBC estimator f˜TS,B (x) with the MISE-optimal c ∗ = 0.2636; and (iii) beta JLN-MBC estimator f˜JLN,B (x).
Smoothing parameters are chosen via the rule-of-thumb method for the transformed data, which picks b̂plain = 0.0323 andb̂mbc = 0.0876.
The three density estimates are plotted in Fig. 3. Note that ‘‘×’’ symbols are data points. Each of three density estimates
successfully captures the bimodal shape. As the true density is unknown, it is hard to judge superiority among threeestimators. However, we can see that compared with the bias-uncorrected estimate, two MBC estimates tend to make theheights of two peaks lower without changing their locations much.

5. Conclusion
This paper has demonstrated that two well-known bias correction techniques can be applied to density estimation
using the beta and modified beta kernels. Under sufficient smoothness of the true density, both bias reduction methods
are shown to improve the order of magnitude in bias from O (b) to O b2 , while the order of magnitude in varianceremains unchanged. Two classes
 of bias-corrected density estimators are by construction nonnegative, and establish a fasterconvergence rate of O n−8/9 in MSE for the interior part when best implemented, as with symmetric second-order kernels.
Monte Carlo simulations indicate superior performance of JLN-type estimators in particular, compared to corresponding
bias-uncorrected estimators.

486

M. Hirukawa / Computational Statistics and Data Analysis 54 (2010) 473–495

0.0

0.8

Density
1.2 1.6

2.0

2.4<< beta-referenced >>0.0 0.4

0.0 0.4

0.8

Density
1.2 1.6

2.0

2.4<< rule-of-thumb >>

0.1

0.2

0.3

0.4

0.5
x

0.6

0.7

0.8

0.9

1.0

0.0

0.1

0.2

0.3

0.4

0.5
x

0.6

0.7

0.8

0.9

1.0

0.8

0.9

1.0

0.8

0.9

1.0

0.8

0.9

1.0

0.0

Density
0.1

0.2

0.3

0.4

0.5
x

0.6

0.7

0.8

0.9

1.0

<< beta-referenced >>0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5<< rule-of-thumb >>0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5

Density(a) Truncated Exp(1/2) [Distribution 3].

0.0

0.1

0.2

0.3

0.4

0.5
x

0.6

0.7

(b) (1/2)B(3, 1) + (1/2)B(1, 3) [Distribution 7].

0.0

1.2

Density

1.6

2.0

2.4

<< beta-referenced >>0.0 0.4

0.8

1.2
0.0 0.4

0.8

Density

1.6

2.0

2.4

<< rule-of-thumb >>

0.1

0.2

0.3

0.4

0.5
x

0.6

0.7

0.8

0.9

0.0

1.0

0.1

0.2

0.3

0.4

0.5
x

0.6

0.7

(c) B(2, 4) [Distribution 8].

0.0

1.6

2.0

2.4

<< beta-referenced >>0.0 0.4

0.8

1.2

Density

1.2
0.0 0.4

0.8

Density

1.6

2.0

2.4

<< rule-of-thumb >>

0.1

0.2

0.3

0.4

0.5
x

0.6

0.7

0.8

0.9

1.0

0.0

0.1

0.2

0.3

0.4

(d) (1/4)B(8, 2) + (3/4)B(2, 8) [Distribution 10].
Fig. 1. Average plots of MBC beta density estimates (n = 100).

0.5
x

0.6

0.7

Density
0.2

0.3

0.4

0.5
x

0.6

0.7

0.8

0.9

1.0

0.0

0.1

0.2

0.3

0.4

0.5
x

0.6

0.7

0.8

0.9

1.0

0.7

0.8

0.9

1.0

(b) Truncated N (0, 1/4) [Distribution 4].

0

0.4

2

0.8

4

DensityDensity
6
8

2.0

10

12

2.4

(a) B(3, 1) [Distribution 2].

0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8

0.1

1.6

0.0

487

1.2

Density

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

M. Hirukawa / Computational Statistics and Data Analysis 54 (2010) 473–495

0.0

0.1

0.2

0.3

0.4

0.5
x

0.6

0.7

0.8

0.9

1.0

(c) B(1/2, 1/2) [Distribution 6].

0.0

0.1

0.2

0.3

0.4

0.5
x

0.6

(d) (1/2)B(3, 1) + (1/2)B(1, 3) [Distribution 7].

1.00
0.00

0.25

0.50

0.75

Density

1.25

1.50

1.75

2.00

Fig. 2. Average plots of ‘‘micro’’ beta density estimates (n = 100; rule-of-thumb method).

1.75

2.25

2.75

3.25

3.75

4.25

4.75

5.25

Eruption Duration (minutes)
Fig. 3. Density estimates of eruption durations of the Old Faithful Geyser.

Appendix
A.1. Proof of Theorem 1
We present only the proof when the beta kernel is employed. The proof when the modified beta kernel is used is similar,
and thus it is omitted. Accordingly, we can safely suppress the subscript j.

488

M. Hirukawa / Computational Statistics and Data Analysis 54 (2010) 473–495

A.1.1. Biasd

The proof largely follows TS. Let θx = B (x/b + 1, (1 − x) /b + 1). Taking a fourth-order Taylor expansion around θx = x
for

n

o

Ib (x) = E fˆb (x) =Z 1

KB(x/b+1,(1−x)/b+1) (u) f (u) du = E {f (θx )} ,0

we haveIb (x) = f (x) +4Xf (j) (x)

j!

j =1

E (θx − x)j + o E (θx − x)4 .



By the property of the beta random variables and a Taylor expansion around b = 0,
E (θx − x) = (1 − 2x) b − 2(1 − 2x)b2 + O b3 ,E (θx − x)2 = x (1 − x) b + (11x2 − 11x + 2)b2 + O b3 ,E (θx − x)3 = 5x (1 − x) (1 − 2x) b2 + O b3 ,E (θx − x)4 = 3x2 (1 − x)2 b2 + O b3 .

An argument similar to the proof of LemmaB2(a)-(ii) in Gospodinov and Hirukawa (2007) can also demonstrate thatE (θx − x)r = O b3 for r ≥ 5. Hence,Ib (x) = f (x) 1 +a1 (x)f (x)
a2 ( x ) 2
b + o b2 ,f ( x)


b+

(1)

where
a1 (x) = (1 − 2x) f 0 (x) +12

x (1 − x) f 00 (x) ,1a2 (x) = −2 (1 − 2x) f 0 (x) +2

511x2 − 11x + 2 f 00 (x) +6

x (1 − x) (1 − 2x) f 000 (x) +1 2
x (1 − x)2 f 0000 (x) .
8Taking the logarithm on both sides of (1) for an arbitrarily small b > 0 and then using a Taylor expansion, we havelog Ib (x) = log f (x) +a1 ( x )

b+

f (x)

n

1



2a2 (x) f (x) − a21 (x)f 2 ( x)2

b2 + o b2 .



(2)

o

Similarly, log Ib/c (x) = E fˆb/c (x) can be approximated bylog Ib/c (x) = log f (x) +1

a1 (x)


b+

f (x)

c

1



2a2 (x) f (x) − a21 (x)

2c 2



( x)

f2

b2 + o b2 .



(3)

Then,
1
1−c

log Ib (x) −c1−clog Ib/c (x) = log f (x) −(1a2 (x) f (x) − 12 a21 (x)c (1 − c ))f 2 (x)b2 + o b2 ,which yields
1 − c{Ib (x)} 1−c Ib/c (x) 1−c = f (x) +

1

  2
1

a1 ( x )c (1 − c )

2

f (x)





− a2 ( x ) b 2 + o b 2(4)after taking the exponential on both sides and using a Taylor expansion.
Define Z = fˆb (x) − Ib (x) and W = fˆb/c (x) − Ib/c (x). Then, E (Z ) = E (W ) = 0, and each of E Z 2
E W2





n
o
= v ar fˆb (x) ,

n
o
n
o

= v ar fˆb/c (x) and E (ZW ) = cov fˆb (x) , fˆb/c (x) is at most O n−1 b−1 , as shown in the variance part below.

Again, by a Taylor expansion,1
˜fTS (x) = {Ib (x)} 1−c 1 +1Z
Ib ( x )= {Ib (x)} 1−c Ib/c (x) 1−1 c

c
− 1−
c

+

Ib/c (x)



1
1−c

c
− 1−
c


Z


1+

Ib (x)
Ib/c (x)− 1−c c

W
Ib/c (x)

c

 1−c
−

c
1−c


W

Ib (x)
Ib/c (x) 1−1 c
+ O (Z + W )2 .

(5)

M. Hirukawa / Computational Statistics and Data Analysis 54 (2010) 473–495

489

Since (2) and (3) imply that Ib (x) /Ib/c (x) = 1 + O (b), substituting (4) finally yields


1 
− c
= {Ib (x)} 1−c Ib/c (x) 1−c + O n−1 b−1
  2




1
1 a1 (x)= f (x) +− a2 (x) b2 + o b2 + O n−1 b−1 ,c (1 − c ) 2 f (x)where the remainder term O n−1 b−1 = o b2 by (A2).
no

E f˜TS (x)A.1.2. Variance
It follows from (5) that


2
n
o

1
c
v ar f˜TS (x) = E
Z−W+ O n− 1
1−c
1−c
h
n
o
n
o
n
oi

1
ˆb (x) − 2c · cov fˆb (x) , fˆb/c (x) + c 2 · v ar fˆb/c (x) + O n−1 .
v
ar
f
=
2
(1 − c )For interior x,

n
o

f (x)
v ar fˆb (x) = n−1 b−1/2 √ √+ o n−1 b−1/2 ,2 π x (1 − x)
n
o

f (x)
v ar fˆb/c (x) = n−1 b−1/2 c 1/2 √ √+ o n−1 b−1/2 .
2 π x (1 − x)
Now,

n

o

cov fˆb (x) , fˆb/c (x) = n−1 E KB(x/b+1,(1−x)/b+1) (X1 ) KB(cx/b+1,c (1−x)/b+1) (X1 ) + O n−1 .

In particular,
E KB(x/b+1,(1−x)/b+1) (X1 ) KB(cx/b+1,c (1−x)/b+1) (X1 ) = Gb (x) {f (x) + o (1)} ,



where


+ 1, (1+c )(b 1−x) + 1
Gb (x) = x
B bx + 1, 1−+ 1 B cxb + 1, c (1b−x) + 1
b

 



0 (1+bc )x + 1 0 (1+c )(b 1−x) + 1 0 1b + 2 0 bc + 2
.
=



 
0 1+b c + 2 0 bx + 1 0 1−b x + 1 0 cxb + 1 0 c (1b−x) + 1
B

(1+c )xb√(6)

Define R (z ) = 2π z z +1/2 exp (−z ) /0 (z + 1) for z > 0, as in Lemma 3 of Brown and Chen (1999). Then, following theargument in Chen (2000a, p. 87),
Gb (x) =



1+c

1/2

b1/2

√

√

x
b

1−x
b

2π

c

1 + 1bx (1 − x)

1 + bcc
1 + 1+b

S ( b, x) ,whereS (b, x) =RR

1 + 1b



1+c
b



R

(1+c )x

1 + bcc
1 + 1+
b




=

R

 

b





R



cx
b

R

(1+c )(1−x)



b

c
1+c



  c (1−x) 
R

R

b


1
b

R

c
b

 → 1,

b−1 {1 + O (b)} .

Therefore,√no

cov fˆb (x) , fˆb/c (x) = n−1 b−1/2

2c 1/2
1/2(1 + c )which establishes the result for interior x.

f (x)+ o n−1 b−1/2 ,√ √2 π x ( 1 − x)

490

M. Hirukawa / Computational Statistics and Data Analysis 54 (2010) 473–495

On the other hand, as x/b → κ when x → 0,no
0 (2κ + 1) f (x)+ o n− 1 b − 1 ,v ar fˆb (x) = n−1 b−1 2κ+1 2
20 (κ + 1)no
0 (2c κ + 1) f (x)v ar fˆb/c (x) = n−1 b−1 c 2c κ+1 2+ o n−1 b−1 .
20 (c κ + 1)
It follows from (6) thatGb (x) ∼∼1(1 + c )κ(1 + c )

c
1+c



1

c κ

κ

0 ((1 + c ) κ + 1)
0 (κ + 1) 0 (c κ + 1)

c κ+1c

1 + 1b

1+

1 + bc R



1+c
b

R

1 +c
b





R

1 −x
b

(1+c )(1−x)
b  c (1−x) 
R

R

b
1
b



0

c
b



0 ((1 + c ) κ + 1) b−1.
0 (κ + 1) 0 (c κ + 1)

1+cTherefore,

n



1

o

cov fˆb (x) , fˆb/c (x) = n−1 b−1κ(1 + c )c κ+1c
1+c
0 ((1 + c ) κ + 1) f (x)+ o n−1 b−1 ,0 (κ + 1) 0 (c κ + 1)

which yields the stated result. The proof for (1 − x) /b → κ is similar.



A.2. Proof of Theorem 2
Again, only the proof when the beta kernel is employed is given. Because JLN-MBC uses a single smoothing parameter,
the subscript b is also suppressed throughout.
A.2.1. BiasPn
The proof largely follows JLN. Let α̂ (x) = n−1 i=1 KB(x/b+1,(1−x)/b+1) (Xi ) /fˆ (Xi ) and rewrite f˜JLN (x) as(f˜JLN (x) = fˆ (x) α̂ (x) = f (x) 1 +fˆ (x) − f (x)f ( x))
1 + α̂ (x) − 1







.

Then,

" (
)
(
!
)#
o


fˆ (x) − f (x)fˆ (x) − f (x)˜
E fJLN (x) = f (x) + f (x) E+ E α̂ (x) − 1 + E
α̂ (x) − 1.
f ( x)
f (x)

n

(7)

We provide approximations to three expectations inside the brackets separately. First, we approximate E α̂ (x) − 1 . Bya geometric expansion,
1 X KB(x/b+1,(1−x)/b+1) (Xi )nα̂ (x) =f (Xi )

n i =11 −where the remainder term is op b2 + n−1 b−1
conditional on Xi . Using (1),(E

fˆ (Xi ) − f (Xi )f ( Xi ))Xi=

a1 (Xi )f (Xi )b+(fˆ (Xi ) − f (Xi )f (Xi ))(+fˆ (Xi ) − f (Xi )f ( Xi ))2  + op b2 + n−1 b−1 ,= op b2 by (A2). We now evaluate the expectation of the ith summand
a2 (Xi ) 2
b + o b2 ≡ h1 (Xi ) b + h2 (Xi ) b2 + o b2 ,f (Xi )

where a1 (x) and a2 (x) are defined in the proof of Theorem 1. In addition, by (A2),(E

fˆ (Xi ) − f (Xi )f (Xi )

)2Xi  = h21 (Xi ) b2 + o b2 + O n−1 b−1 = h21 (Xi ) b2 + o b2 .

Hence, the conditional expectation of the ith summand can be approximated byKB(x/b+1,(1−x)/b+1) (Xi ) 
1 − h1 (Xi ) b + h21 (Xi ) − h2 (Xi ) b2 + o b2 .
f (Xi )

(8)

M. Hirukawa / Computational Statistics and Data Analysis 54 (2010) 473–495

491

It follows thatE α̂ (x) = EKB(x/b+1,(1−x)/b+1) (X1 )f (X1 )


+E


−E

KB(x/b+1,(1−x)/b+1) (X1 )f (X1 )

KB(x/b+1,(1−x)/b+1) (X1 )  2
h1 (X1 ) − h2 (X1 )f (X1 )

h1 (X1 ) b

b2 + o b2= 1 − E {h1 (θx )} b + E h21 (θx ) − h2 (θx ) b2 + o b2 ,dwhere θx = B (x/b + 1, (1 − x) /b + 1). By the property of the beta random variables and a Taylor expansion around θx = x,
we have

E {h1 (θx )} = h1 (x) + (1 − 2x) h01 (x) +E h1 (θx ) − h2 (θx ) =

 2

h21

1
2



x (1 − x) h001 (x) b + o (b) ,(x) − h2 (x) + O (b) ,so that






1
00
2
0
b2 + o b2 .
E α̂ (x) = 1 − h1 (x) b + h1 (x) − h2 (x) − (1 − 2x) h1 (x) + x (1 − x) h1 (x)(9)

2Second, it immediately follows from (8) that(Efˆ (x) − f (x)

)

f (x)


= h1 (x) b + h2 (x) b2 + o b2 .

(10)

Third, by the Cauchy–Schwarz inequality and (A2),"(Efˆ (x) − f (x))#
α̂ (x) − 1

f ( x)

= −h21 (x) b2 + o b2 .

(11)

Finally, substituting (9)–(11) into (7) yieldsno

E f˜JLN (x)1= f (x) − f (x) (1 − 2x) h01 (x) + x (1 − x) h001 (x) b2 + o b2
2




0
00 

a1 (x)
a1 ( x )1= f (x) − f (x) (1 − 2x)+ x ( 1 − x)b2 + o b2 .
f (x)
2
f (x)
A.2.2. VarianceObserve that(f˜JLN (x) = f (x) 1 +fˆ (x) − f (x)

)

f (x)

1 X KB(x/b+1,(1−x)/b+1) (Xi )n

1 X KB(x/b+1,(1−x)/b+1) (Xi )n∼ f (x)f (Xi )

n i =1f (Xi )

n i=1

(
2−

fˆ (Xi )f ( Xi )"1−fˆ (Xi ) − f (Xi )f (Xi )

(
+o

)
,

where
n
X
KB(x/b+1,(1−x)/b+1) (Xi )(f (Xi )i =1

2−

fˆ (Xi ))f ( Xi ))nnX
2KB(x/b+1,(1−x)/b+1) (Xi )1 X KB(x/b+1,(1−x)/b+1) Xj KB(Xj /b+1,(1−Xj )/b+1) (Xi )=−f ( Xi )n j =1f 2 Xj
i=1
(

≡

n
X

{ς1 (Xi ) − ς2 (Xi )}

i=1

≡

n
X
i=1

ς (Xi ) .

fˆ (Xi ) − f (Xi )f (Xi ))#

492

M. Hirukawa / Computational Statistics and Data Analysis 54 (2010) 473–495

We approximate ς2 (Xi ) for a given data point Xi . The idea closely follows Section 3.2 of Jones and Henderson (2007b).
For Xi 6= 0, 1,

ς2 (Xi ) ∼ ϕ (Xi )(KB(x/b+1,(1−x)/b+1) Xj KB(Xj /b+1,(1−Xj )/b+1) (Xi )≡Ef 2 Xj

Z 1

KB(x/b+1,(1−x)/b+1) (u) KB(u/b+1,(1−u)/b+1) (Xi )0f ( u)
 u


1 −u

b

b

=

1−x

x

u b (1 − u) b exp

Z 1=

log Xi +
x
b

B

0

)
Xi

du

log (1 − Xi ) − log Bub+ 1, 1−b u + 1

+ 1, 1−b x + 1 f (u)du.

By Stirling’s approximation,


log B

u
b

1−u

+ 1,

b


1
1
+ 1 = {u log u + (1 − u) log (1 − u)} + log b
b

2

+

1

log 2π +2

1

log u (1 − u) −2

b u2 − u + 1+ O b3/2 ,12u (1 − u)and thus
b−1/2

Z 1

2π

0

ϕ (Xi ) = √(ψ (u) exp

u

log

b

Xi
u


+

1−u
b




log

1 − Xi1−u−b u2 − u + 1
12u (1 − u)

+O b

)

3/2

du,

where
x

ψ (u) =1−x

11

u b − 2 (1 − u) b − 2
f (u) B

x
b

+ 1,

1 −x
b

KB(x/b+1,(1−x)/b+1) (u) =√u (1 − u)f (u)+1.

By a change of variable w = (Xi − u) /b1/2 and a Taylor expansion,

Z Xi /b1/2

w2
ψ Xi − bϕ (Xi ) = √2Xi (1 − Xi )2π −(1−Xi )/b1/2
(
)


2
1/2
3

b 3Xi − 3Xi + 1 w 4
b Xi2 − Xi + 1
b (1 − 2Xi ) wb (1 − 2Xi )2 w 6
× 1−
+
−
−
+ O b3/2 dw.
12Xi (1 − Xi )6Xi2 (1 − Xi )2
12Xi3 (1 − Xi )3
72Xi4 (1 − Xi )4
1

1/2


w exp −
√

By another change of variable v = w/ Xi (1 − Xi ),ϕ (Xi ) =p

Xi (1 − Xi )

(
×
∼

1−

Z √Xi /(1−Xi )/b1/2√− (1−Xi )/Xi /b1/2

b1/2 (1 − 2Xi ) v 3√

6 Xi (1 − Xi )KB(x/b+1,(1−x)/b+1) (Xi )f (Xi )b 3Xi2 − 3Xi + 1 v 4



−


×



p
ψ Xi − b1/2 Xi (1 − Xi )v φ (v)1Φ

12Xi (1 − Xi )

√ 
κ



−

b Xi2 − Xi + 1
12Xi (1 − Xi )+b (1 − 2Xi )2 v 6
72Xi (1 − Xi )if Xi /b → ∞ and (1 − Xi ) /b → ∞if Xi /b → κ or (1 − Xi ) /b → κ,

where φ (·) and Φ (·) denote the standard normal density and distribution functions. Therefore,ς (Xi ) ∼ ς1 (Xi ) − ϕ (Xi )∼KB(x/b+1,(1−x)/b+1) (Xi )f ( Xi )


×

1
√ 
2−Φ
κ

if Xi /b → ∞ and (1 − Xi ) /b → ∞if Xi /b → κ or (1 − Xi ) /b → κ≡ ζ (Xi ) .
It follows that



n
o

1
1 
v ar f˜JLN (x) ∼ f 2 (x) v ar {ζ (X1 )} = f 2 (x) E ζ 2 (X1 ) + O n−1 .
n

n

)
+O b

3/2



dv

M. Hirukawa / Computational Statistics and Data Analysis 54 (2010) 473–495

Pick δ = b1− where  ∈ (0, 1/2). Following the trimming argument as in Chen (1999),
E ζ 2 (X1 ) =Z 1ζ 2 (u) f (u) du

0

Z δ

Z 1−δ

Z 1
+

+

=

δ

0

= Ab (x)Z 1

ζ 2 (u) f (u) du

1−δKB(2x/b+1,2(1−x)/b+1) (u)
f (u)

0

du + O b− ,



where
B

Ab (x) =



2x
b

B2


b−1/2


 √ √
+ 1,
+1
 ∼ 2 π x (1 −−x1)
1 −x
x

+
1
,
+
1
 0 (2κ + 1) bbb
22κ+1 0 2 (κ + 1)2(1−x)b

if x/b → ∞ and (1 − x) /b → ∞if x/b → κ or (1 − x) /b → κ.

The stated result follows from recognizing that

Z 1

KB(2x/b+1,2(1−x)/b+1) (u)
f (u)

0

du = E f −1 (ϑx ) = f −1 (x) + O (b) ,dwhere ϑx = B (2x/b + 1, 2 (1 − x) /b + 1).



A.3. Formulae for beta-referenced smoothing parameters
The analytical expression of b̂TS,MB isb̂TS,MB = c (1 − c ) λ (c )

 2

2

2/9



B (α, β) B (α + 9/2, β + 9/2)√

16 π CTS,MB (α, β)2/9

n−2/9 ,

where
CTS,MB (α, β) =

ψ1 =
ψ2 =

1

1

ψ 2 0 (2α) 0 (2β + 8) − 2ψ1 ψ2 0 (2α + 1) 0 (2β + 7)
0 (2α + 2β + 8) 1+ ψ22 + 2ψ1 ψ3 0 (2α + 2) 0 (2β + 6) − 2 (ψ1 ψ4 + ψ2 ψ3 ) 0 (2α + 3) 0 (2β + 5)+ ψ32 + 2ψ1 ψ5 + 2ψ2 ψ4 0 (2α + 4) 0 (2β + 4)
− 2 (ψ2 ψ5 + ψ3 ψ4 ) 0 (2α + 5) 0 (2β + 3) + ψ42 + 2ψ3 ψ5 0 (2α + 6) 0 (2β + 2)
− 2ψ4 ψ5 0 (2α + 7) 0 (2β + 1) + ψ52 0 (2α + 8) 0 (2β) ,(α − 1)2 (α − 2)2 −

8
1
2



1
3

(α − 1) (α − 2) (α − 3) −(α − 1)2 (α − 2) (β − 1) −− (α − 1) (α − 2) (β − 1) −
1

1
2
1
2

(α − 1) (α − 2) −

1
8
1
3

(α − 1) (α − 2) (α − 3) (α − 4) ,(α − 1) (α − 2) (α − 3)

(α − 1) (α − 2) (α − 3) (β − 1) ,

ψ3 =1
(α − 1) (α − 2) (β − 1) (β − 2) − (α − 1) (β − 1)2− (α − 1) (α − 2) (β − 1) − (α − 1) (β − 1) (β − 2) ,

ψ4 =

1

2

2

(α − 1)2 (β − 1)2 −(α − 1) (β − 1)2 (β − 2) −− (α − 1) (β − 1) (β − 2) −
ψ5 =

1
8

(β − 1)2 (β − 2)2 −

1
3

1
2
1
2

(β − 1) (β − 2) −

(β − 1) (β − 2) (β − 3) −B (α, β) B (α + 9/2, β + 9/2)√
4 π CJLN,MB (α, β)3(β − 1) (β − 2) (β − 3)

(α − 1) (β − 1) (β − 2) (β − 3) ,Furthermore, b̂JLN,MB takes the form ofb̂JLN,MB =

1

2/9

n−2/9 ,

18

(β − 1) (β − 2) (β − 3) (β − 4) .

493

494

M. Hirukawa / Computational Statistics and Data Analysis 54 (2010) 473–495

where
CJLN,MB (α, β) =1

(α − 1)2 (α − 2)20 (2α + 2β + 8)
× {0 (2α) 0 (2β + 8) + 20 (2α + 1) 0 (2β + 7) + 0 (2α + 2) 0 (2β + 6)}+ 2 (α − 1) (α − 2) (β − 1) (β − 2)
× {0 (2α + 3) 0 (2β + 5) + 20 (2α + 4) 0 (2β + 4) + 0 (2α + 5) 0 (2β + 3)}+ (β − 1)2 (β − 2)2 {0 (2α + 6) 0 (2β + 2) + 20 (2α + 7) 0 (2β + 1) + 0 (2α + 8) 0 (2β)} .
On the other hand, Jones and Henderson (2007b) define the beta-referenced smoothing parameter for fˆMB (x) asnob̂MB = arg min AWMISE fˆMB (x)b= arg min
b

b2
4

Z 1

x (1 − x) g (x)



00

2

v (x) dx +0n−1 b−1/2

Z 1
√

√

2 π

0

g (x)
x (1 − x)v (x) dx,

where the weighting function v (x) is chosen as v (x) = x3 (1 − x)3 to ensure finiteness of integrals. It follows that b̂MB canbe expressed asb̂MB =B (α, β) B (α + 5/2, β + 5/2)√
2 π CMB (α, β)2/5

n−2/5 ,

where
CMB (α, β) =10 (2α + 2β + 4)
(α − 1)2 (α − 2)2 0 (2α) 0 (2β + 4)

− 4 (α − 1)2 (α − 2) (β − 1) 0 (2α + 1) 0 (2β + 3)
+ 2 (α − 1) (β − 1) (3αβ − 4α − 4β + 6) 0 (2α + 2) 0 (2β + 2)
− 4 (α − 1) (β − 1)2 (β − 2) 0 (2α + 3) 0 (2β + 1) + (β − 1)2 (β − 2)2 0 (2α + 4) 0 (2β) . 
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