quantile regression via tpnd - aucklandaard004/arar.pdfefron (1991) introduced the percentile and...
TRANSCRIPT
Quantile Regression via TPND
© A. Ardalan
University of Auckland
30 Aug 2011 @ Auckland
Joint with:T. W. Yee
http://www.stat.auckland.ac.nz/~aard004
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 1 / 31
Contents
Outline of this document
1 Quantile Regression
2 Expectile regression
3 Two-Piece Normal Distribution
4 Quantile Regression via Two-piece normal distributionQuantile Regression by TPNPenalized Iterratively Reweighted Least Square AlgorithmAsymptotic Normality
5 Summary
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 2 / 31
Quantile Regression
Introduction to Quantile Regression ISome motivation
Q: Why quantiles?To begin with, people prefer the summary of every thing.Usually, we summarise the data in central tendencies such as mean,median or even mode.Sometimes these summaries are deceptive. In this regard the quantilesenable us to visualise the shape of the distribution rather than the centraltendencies.In brief, quantiles give us a good picture of the distribution of data.
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 3 / 31
Quantile Regression
Introduction to Quantile Regression IISome motivation
Q: Why quantile regression?
Regression analysis includes any techniques for modeling relationshipbetween dependent variable Y and one or more explanatoryvariables X .
Typically, we use the quadratic loss or absolute loss function and itmeans we look at the E [Y |X ] or Median[Y |X ], respectively.
Sometimes we lose some information and these models are notappropriate for all data. In addition, in some cases the tails of thedistributions are of more interest than the center of them.
To give a more complete picture of the relationship between theresponse and explanatory variables, we can consider the quantiles ofdistribution of [Y |X ] i.e. Q[Y |X ]
The resulting curves are called the quantile regression curves.Clearly, they can be smoothed in some ways.
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 4 / 31
Quantile Regression
Let (X , Y ) have the bivariate normal distribution,
(X ,Y ) ∼ N2(µx , µy , σx , σy , ρ)
the conditional distribution of Y |X has normal distribution
(Y |X = x) ∼ N(µ(Y |x), σ(Y |x)
)where
µ(Y |x) = µy + ρσyσx
(x − µx), σ(Y |x) = σ2y (1− ρ2)
and also
Q(Y |x)(p) = µ(Y |x) + σ(Y |x)Φ−1(p)
where Φ−1(p) is the pth quantile of the standard normal distribution.
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 5 / 31
Quantile Regression
Let (X , Y ) have the bivariate normal distribution,
(X ,Y ) ∼ N2(µx , µy , σx , σy , ρ)
the conditional distribution of Y |X has normal distribution
(Y |X = x) ∼ N(µ(Y |x), σ(Y |x)
)where
µ(Y |x) = µy + ρσyσx
(x − µx), σ(Y |x) = σ2y (1− ρ2)
and also
Q(Y |x)(p) = µ(Y |x) + σ(Y |x)Φ−1(p)
where Φ−1(p) is the pth quantile of the standard normal distribution.
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 5 / 31
Quantile Regression
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© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 6 / 31
Quantile Regression
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75%
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Y
Figure: (X , Y ) have the bivariate normal distributionQ(Y |x)(p) = µ(Y |x) + σ(Y |x)Φ
−1(p)
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 7 / 31
Quantile Regression
age
Den
sity
20 40 60 80
0.0000.0050.0100.0150.0200.0250.030
20 30 40 50 60 70 80
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BM
I
0.00 0.06
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BM
I
Figure: The body mass indexes and ages from a random sample of New Zealandadults, n = 700. BMI = their body mass indexes, which is their weight divided bythe square of their height (kg/m2), age = their age (years).
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 8 / 31
Quantile Regression
age
Den
sity
20 40 60 80
0.0000.0050.0100.0150.0200.0250.030
20 30 40 50 60 70 80 90
20
30
40
50
60
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BM
I
25%50%75%
0.00 0.06
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Density
BM
I
Figure: The body mass indexes and ages from a random sample of New Zealandadults, n = 700. BMI = their body mass indexes, which is their weight divided bythe square of their height (kg/m2), age = their age (years).
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 9 / 31
Quantile Regression
Applications of quantile regression come from many fields. Here aresome:
Medical examples include investigating height, weight, body massindex (BMI) as a function of age of the person.
Economics, e.g., it has been used to study determinants of wages,discrimination effects, and trends in income inequality. SeeKoenker (2005) for more references.
Education, e.g., the performance of students in public schools onstandardized exams as a function of socio-economic variables such asparents’ income and educational attainment.
Ecology, e.g., the Melbourne temperature data exhibits bimodalbehaviour.
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 10 / 31
Quantile Regression
Growth chart example
Girls’ height and weight.
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 11 / 31
Quantile Regression
Three classes
1 Classical Quantile Regression models. Koenker and Bassett (1978)introduced the classical quantile regression estimator. (UsingAsymmetric L1 loss function)
2 Expectile regression methods. Newey and Powell (1987) andEfron (1991) introduced the percentile and expectile quantileregression estimator. (Using Asymmetric L2 loss function).
3 LMS-type methods. These transform the response to someparametric distribution (e.g., Box-Cox to N(0, 1)). Estimatedquantiles on the transformed scale are back-transformed on to theoriginal scale Cole and Green (1992). A problem with the LMSmethod is to find justification for the underlying method.
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 12 / 31
Quantile Regression
Classical Quantile Regression
Koenker and Bassett (1978) considered asymmetric L1 loss function
ρp(u) =
{(1− p)(−u) u < 0,p(u) u ≥ 0,
A specific quantile can be found by minimizing the expected loss of Y − uwith respect to u
minu
E (ρp(Y−u)) = minu
{(p − 1)
∫ u
−∞(y − u)dFY (y) + p
∫ ∞u
(y − u)dFY (y)
}.
This can be shown by setting the derivative of the expected loss functionto 0 and letting qp the pth quantile of the random variable Y .The pth sample quantile can be obtained by solving
qp = arg minq∈R
n∑i=1
ρp(yi−q) = arg minq∈R
(p − 1)∑yi<q
(yi − q) + p∑yi≥q
(yi − q)
.© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 13 / 31
Quantile Regression
Suppose the pth conditional quantile function is Q(Y |X )(p) = Xβ(p).Solving the sample analog gives the estimator of β.
β(p) = arg minβ∈Rk
n∑i=1
(ρp(Yi − xiβ)).
The intuition of this, is the same as solving the population quantile.Quantiles traditionally are estimated by linear programming.Koenker and Machado (1999) considered the following represention ofasymmetric Laplace distribution that can be applied in quantile regression
f (y ;µ, σ, p) =p(1− p)
σexp
{−ρp
(y − µσ
)}(1)
The log-likelihood under the assumption that the εi come from this densityis
`(β) = n log
(p(1− p)
σ
)+ exp
{n∑
i=1
−ρp(
Yi − xiβ
σ
)}(2)
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 14 / 31
Expectile regression
Percentile and Expectile Regression
Newey and Powell (1987) have considered the asymmetric quadratic lossfunction as an alternative of asymmetric L1 loss function and they defineda new concept and they called it expectile
ρ[2]ω (u) =
{(1− ω)(u)2 u < 0,ω(u)2 u ≥ 0,
An expectile is the minimization of the quantity E[ρ[2]ω (Y − µ)
]wrt µ.
In fact, it is a generalization of mean.Expectiles are similar to quantiles except that they are defined by tailexpectations; see Newey and Powell (1987).The ωth sample expectile can be obtained by solving
µω = arg minµ∈R
n∑i=1
ρ[2]ω (yi − µ)2,
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 15 / 31
Expectile regression
Consider the linear model
yi = xiβ + εi , for i = 1, . . . , n.
Letri (b) = yi − xTi β
be a residual.We would like to compute β
(α)by minimizing
Sω(b) =∑n
i=1 ρ[2]ω {ri (b)}.
We can represent Sω(b) as a matrix weighted model,
Sw (b) = (y − Xb)TW(b)(y − Xb),
where w = ω1−ω and W(b) = diag[w(ri (b))] ia a diagonal matrix and,
w(ri (β)) =
{1 ri (b) ≤ 0,w ri (b) > 0,
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 16 / 31
Expectile regression
Sw (b) is strictly convex and continuously differentiable as a function of b,see Efron (1991). This implies that the minimizer βw exists uniquely, andequals to solution of
Sw (b) ≡ ∇bSw (b) = 0,
and β therefore is the solution of
Sw (b) = 0
and iterative methods are needed to actually solve.The second derivative of Sw (b) is
Sw (b) ≡(∂2Sw (b)
∂bj∂bj ′
)j ,j ′=1,2,...,K
Efron (1991): the usual Newton-Raphson formula suggests bNEW thesolution of Sw (b) = 0, where
bNEW − b = −S−1w (b)Sw (b)
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 17 / 31
Expectile regression
−2 −1 0 1 2
0.0
0.5
1.0
1.5
(a)
x
Loss
Fun
ctio
n
ABSA−ABS
y = x
−2 −1 0 1 2
0.0
0.5
1.0
1.5
2.0
(b)
x
Loss
Fun
ctio
n
SquaredA−Squared
−2 −1 0 1 2
−1.5
−1.0
−0.5
0.0
0.5
1.0
1.5
(c)
x
Influ
ence
Fun
ctio
n
−2 −1 0 1 2
−3
−2
−1
0
1
2
3
(d)
x
Influ
ence
Fun
ctio
n
Figure: Loss functions for (a) quantile regression with p = 0.5 (L1 regression) andp = 0.75 (asymmetric absolute loss function); (b) expectile regression withω = 0.5 (least squares) and ω = 0.75 (asymmetric least squares). (c) and (d) arederivatives of loss functions or Influence functions of (a) anb (b) respectively.
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 18 / 31
Two-Piece Normal Distribution
Two-piece normal distributionDistribution properties
The two-piece normal distribution (TPN) has a density
f (y ; p, µ, σ) =1
σ√
2π
exp
{− (y−µ)2
8p2σ2
}, y ≤ µ,
exp{− (y−µ)2
8(1−p)2σ2
}, y > µ
(3)
Here, −∞ < µ <∞ is the location parameter, σ > 0 is the scaleparameter and 0 < p < 1 is the shape parameter.Moreover, µ is the pth quantile of distribution, i.e. P(Y ≤ µ) = p.The expected information matrix (EIM) is,
14p(1−p)σ2 0 −2
σ√π
1p(1−p)
0 2σ2 0
−2σ√π
1p(1−p) 0 3
p(1−p)
. (4)
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 19 / 31
Two-Piece Normal Distribution
−6 −4 −2 0 2 4 6
0.00
0.05
0.10
0.15
0.20
0.25
0.30
x
TP
N D
ensi
ty
P(X ≤ 0) = 0.75P(X > 0) = 0.25
−6 −4 −2 0 2 4 6
0.00
0.05
0.10
0.15
0.20
0.25
0.30
x
TP
N D
ensi
ty
P(X ≤ 0) = 0.25P(X > 0) = 0.75
−2 0 2 4
0.0
0.2
0.4
0.6
0.8
1.0
Blue is density, red is cumulative distribution function
Purple lines are the 10,20,...,90 percentilesx
ftpn(
loca
tion=
0 ,
scal
e= 1
.2 ,
skew
par=
0.2
5 )
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 20 / 31
Quantile Regression via Two-piece normal distribution
Quantile Regression by TPN
Consider the linear model
yi = xiβ + εi , for i = 1, . . . , n.
and letεi ∼ TPN(0, σ, p) for i = 1, . . . , n
L(β) =n∏
i=1
f (yi ;β, σ) =n∑
i=1
ρ
(yi − xtiβ
σ
)(5)
ρ(x) = − log(f (x)) (6)
β(p)n = arg min
{n∑
i=1
ρ
(yi − xtiβ
σ
): β ∈ Rq
}
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 21 / 31
Quantile Regression via Two-piece normal distribution
The matrix representation is
L(β) = (1
σ√
2π)n exp
{− 1
σ2(y − Xβ)TW(y − Xβ)
}(7)
where, W is a diagonal matrix, and the diagonal elements are
w(ii) =
1
8p2(y − Xβ) ≤ 0
18(1−p)2 (y − Xβ) > 0
`(β) = n log(σ)− 1
σ2(y − Xβ)TW(y − Xβ) (8)
U(β) =∂`(β)
∂β= 2
XTW(y − Xβ)
σ2(9)
U(β) =∂2`(β)
(∂β2= −2
XTWX
σ2(10)
U(β) = U(β) + U(β)(β − β) (11)
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 22 / 31
Quantile Regression via Two-piece normal distribution
Iteratively Reweighted Least Square Algorithm
β(new) = β(old) −[U(β)
]−1U(β) (12)
the[U(β)
]−1is not continuous, so we can subtitute it by EIM of TPND.
Then we have (Fisher scoring algorithm)
β(new) = β(old) − [EIM]−1U(β) (13)
and so
β(new) = β(old) + 8σ2p(1− p)(XTX
)(−1)(XTW(old)(y − Xβ(old))
σ2
)=
(XTX
)(−1)XT[Xβ(old) + 8p(1− p)W(old)
(y − Xβ(old))
)]︸ ︷︷ ︸
z
=(XTX
)(−1)XTz (14)
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 23 / 31
Quantile Regression via Two-piece normal distribution
−3 −2 −1 0 1 2 3
−3
−2
−1
0
1
2
3
0.25% quantile regression via TPNL distribution
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25%
Q(Y |x)(0.25) = a(0.25) + b(0.25)x
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 24 / 31
Quantile Regression via Two-piece normal distribution
−2 0 2 4
−2
−1
0
1
2
3
0.75% quantile regression via TPNL distribution
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75%
Q(Y |x)(0.75) = a(0.75) + b(0.75)x
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 25 / 31
Quantile Regression via Two-piece normal distribution
Penalized Iteratively Reweighted Least Square Algorithm
In a similar way we have
β(new) = β(old) +
(NTN
4σ2p(1− p)− λD
)(−1)(NTW(old)(y −Nβ(old))
σ2− λβ(old)D
)=
(NTN
4σ2p(1− p)− λD
)(−1)
NT
[Nβ(old) − 2
σ2W(old)
(y −Nβ(old)
)]︸ ︷︷ ︸
z
=
(NTN
4σ2p(1− p)+ λD
)(−1)
NT z (15)
λ is the smoothed parameter.
N is natural cubic spline matrix.
D is the penalty term.
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 26 / 31
Quantile Regression via Two-piece normal distribution
10 20 30 40 50
10
15
20
25
30
35
40
yesterday
toda
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Figure: Melbourne temperature data (◦C). These are daily maximumtemperatures during 1981–1990, n = 3650. Y = each day’s maximumtemperature, X = the previous day’s maximum temperature.
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 27 / 31
Quantile Regression via Two-piece normal distribution
10 20 30 40 50
10
15
20
25
30
35
40
yesterday
toda
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15 %25 %
35 %
45 %
55 %
65 %
75 %
85 %
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 28 / 31
Quantile Regression via Two-piece normal distribution
Asymptotic NormalityConsider,
B1) max1≤k≤n
x′nk(X′nXn)−1xnk → 0 as n→ 0, (16)
B2) limn→∞
n−1(X′nXn)−1 = V, finite and p.d. (17)
Then we haveTheorem: As n→∞, under the assumptions B1 and B2, the limiting
distribution of√
n(β(p)n − β(p)) is a multivariate normal distribution with
variance-covariance matrix 4σ2p(1− p)V−1.
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 29 / 31
Summary
Summary
In this talk we have briefly explored QR.
We introduce the two piece normal distribution which it is anasymmetric distribution.
One of the applications of this family of distribution is in quantileregression and it can be a good alternative for expectile regression (L2
asymmetric loss) and since the location of it is the pth quantile canbe a good alternative for classical quantile regression (Asymmetric L1)in some cases.
However this method is not robust.
A package is under construction.
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 30 / 31
Summary
Thank You
© A. Ardalan (University of Auckland ) Quantile Regression via TPND 30 Aug 2011 @ Auckland 31 / 31