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Bootstrap approximation of wavelet estimates in a semiparametric...
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半参数回归模型中小波估计的Bootstrap逼近,薛留根,刘强,本文使用小波和Bootstrap方法研究了半参数回归模型中参数的推断. 利用Efron再抽样技术构造了Bootstrap统计量, 并证明了Bootstrap逼近的强一致
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Bootstrap approximation of wavelet estimates in a semiparametric
regression model
Liugen Xue Qiang Liu
College of Applied Sciences School of Statistics
Beijing University of Technology Capital University of Economics and Business
Beijing, P. R. China 100124 Beijing, P. R. China 100070
lgxue@bjut.edu.cn liuqiang2008@emails.bjut.edu.cn
Abstract
The inference for the parameters in a semiparametric regression model is stud-
ied by using the wavelet and the bootstrap methods. The bootstrap statistics
are constructed by using Efron’s resampling technique, and the strong uniformly
convergency of the bootstrap approximation is proved. Our results can be used
to construct the large sample confidence intervals for the parameters of interest.
A simulation study is conducted to evaluate the finite-sample performance of
the bootstrap method and to compare it with the normal approximation-based
method.
Keywords: Bootstrap approximation; Confidence interval; Semiparametric re-
gression model; Strong uniformly convergency; Wavelet estimate.
1 Introduction
The wavelet smoothing is an useful method for constructing the nonparametric
estimates. It has many nice properties, the main advantage is that it has the low
demand for the function of interest and obtains nice properties of large sample. Many
authors have applied the wavelet smoothing to the estimation of density function, the
estimation of nonparametric regression function and the semiparametric regression
models. Some related works include: Antoniads Gregorie (1994), Donob (1996),
Speckman (1988), Chai and Xu (1999), Qian and Chai (1999) and Xue (2002).
Consider the semiparamertic regression model
y
i
= x
0
i
β + g(t
i
) + e
i
, i = 1, . . . , n, (1.1)
0
The research was supported by the National Natural Science Foundation of China
(No.10571008), the Natural Science Foundation of Beijing (No.1072004) and Ph. D. Program
Foundation of Ministry of Education of China (No. 20070005003).
1
http://www.paper.edu.cn
where β = (β
1
, . . . , β
d
)
0
is an unknown parametric vector, g(·) is an unknown func-
tion on [0, 1], {x
i
= (x
i1
, . . . , x
id
)
0
; 1 ≤ i ≤ n} are the random design sequences,
{t
i
; 1 ≤ i ≤ n} are the constant sequences on [0,1], and the errors e
i
are independent
and identically distributed (i.i.d.) random variables with mean 0 and variance σ
2
.
Model (1.1) was proposed by Engle et al. (1986), and has been widely investi-
gated in recent years. Some estimation methods have been developed, such as the
kernel smoothing,the wavelet smoothing, the piecewise polynomial method, and so
on. See, for example, Speckman (1988), Chai and Xu (1999) and Chen (1988). They
obtained that the estimator of β achieved the convergence rate O
P
(n
−1/2
). However,
it is very important to study the bootstrap approximation of the estimator.
Efron (1979) introduced the b ootstrap method for estimating the distribution
of statistic based on independent observations. The distribution is obtained by
replacing the unknown distribution with the empirical distribution of the data in the
definition of the statistical function, and then resample the data to obtain a Monte
Carlo distribution for the resulting random variable. This method would probably
be used in practice only when the distribution could not be estimated analytically.
The purpose of the present investigation is to construct the bootstrap statistics of the
wavelet estimators of β and σ
2
for model (1.1) using Efron’s resampling procedure.
We also prove the consistency of the bo otstrap approximation. Our results can be
used to construct the large sample confidence intervals for β and σ
2
.
The rest of this paper is organized as follows. In section 2, the wavelet estimators
of β and σ
2
,
ˆ
β and ˆσ
2
say, are constructed by combining the wavelet smoothing and
the least square method, and the bootstrap statistics of
ˆ
β and ˆσ
2
are given by using
bootstrap method. Section 3 gives the main results. In section 4, a simulation study
is carried out to compare the proposed method with the normal approximation-based
method. The proofs of theorems appear in the appendix.
2 Estimation method
Using the condition of Speckman (1988), we assume, as is common in the setting
of semiparametric regression model, that {x
i
} and {t
i
} are related via
x
ir
= f
r
(t
i
) + η
ir
, 1 ≤ r ≤ d, i = 1, . . . , n,
where f
r
(t) is a given function on [0,1], {η
i
= (η
i1
, . . . , η
id
)
0
; 1 ≤ i ≤ n} are i.i.d.
random variables with independent of {e
i
; 1 ≤ i ≤ n}, and satisfy E(η
i
) = 0 and
Cov(η
i
) = V . Here V = (v
ij
) is a d × d positive definite matrix.
To construct the wavelet estimators of β and σ
2
, we take the scalar function φ(·)
on the Schwarz space S
l
of order l. The multi-resolving analysis of connection L
2
(R)
is {V
m
, m ∈ Z} , the regenerated kernel of V
m
is E
m
(t, s), where
E
m
(t, s) = 2
m
E
0
(2
m
t, 2
m
s) = 2
m
X
k∈Z
φ(2
m
t − k)φ(2
m
s − k).
2
http://www.paper.edu.cn
Let A
i
= [s
i−1
, s
i
), where s
0
= 0, s
n
= 1 and s
i
= (t
i
+ t
i+1
)/2, i = 1, . . . , n − 1.
Denote
˜
X = (˜x
1
, . . . , ˜x
n
)
0
n×d
and ˜y = (˜y
1
, . . . , ˜y
n
)
0
, where
˜x
i
= x
i
−
n
X
j=1
x
j
Z
A
j
E
m
(t
i
, s)ds, ˜y
i
= y
i
−
n
X
j=1
y
j
Z
A
j
E
m
(t
i
, s)ds.
Combining the wavelet and the least square methods, we can define the estimators
of β and σ
2
respectively, that is
ˆ
β = (
˜
X
0
˜
X)
−1
˜
X
0
˜y
and
ˆσ
2
=
1
n
n
X
i=1
(˜y
i
− ˜x
0
i
ˆ
β)
2
≡
1
n
n
X
i=1
ˆe
2
i
,
where ˆe
i
= ˜y
i
− ˜x
0
i
ˆ
β.
To simulate the distributions of
√
n(
ˆ
β−β) and
√
n(ˆσ
2
−σ
2
), we use the bootstrap
method introduced by Efron (1979). The method had been widely applied in some
statistical problem such as the parametric estimation. We use Efron’s resampling
technique to construct the bootstrap statistics of
ˆ
β and ˆσ
2
,
ˆ
β
∗
and ˆσ
∗2
say. Denote
ˆµ =
1
n
P
n
i=1
ˆe
i
. Let F
∗
n
be the empirical distribution of ˆe
i
− ˆµ, so F
∗
n
puts mass 1/n
at ˆe
i
− ˆµ. Given (x
1
, y
1
), . . . , (x
n
, y
n
), let e
∗
1
, . . . , e
∗
n
be conditionally independent
with common distribution F
∗
n
. Write y
∗
i
= ˜x
0
i
ˆ
β + e
∗
i
and y
∗
= (y
∗
1
, . . . , y
∗
n
)
0
. The
bootstrap statistics of
ˆ
β and ˆσ
2
are defined as
ˆ
β
∗
= (
˜
X
0
˜
X)
−1
˜
X
0
y
∗
and
ˆσ
∗2
=
1
n
n
X
i=1
(y
∗
i
− ˜x
0
i
ˆ
β
∗
)
2
≡
1
n
n
X
i=1
ˆe
∗2
i
,
where ˆe
∗
i
= y
∗
i
−
˜
X
0
i
ˆ
β
∗
= [I
n
−
˜
X(
˜
X
0
˜
X)
−1
˜
X
0
e
∗
i
], i = 1, . . . , n.
We can use the distributions of
√
n(
ˆ
β
∗
−
ˆ
β) and
√
n(ˆσ
∗2
− ˆσ
2
) to simulate the
distributions of
√
n(
ˆ
β − β) and
√
n(ˆσ
2
− σ
2
) respectively.
3 Main results
In this section, we first give the asymptotic properties of the bootstrap statistics
of
ˆ
β and ˆσ
2
, and then construct the confidence intervals of β and σ
2
.
3
http://www.paper.edu.cn
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