Tests for validity of the semiparametric heteroskedastic transformation model

Journal Pre-proof Tests for validity of the semiparametric heteroskedastic transformation model Marie Hušková, Simos G. Meintanis, Charl Pretorius

PII: DOI: Reference:

S0167-9473(19)30250-6 https://doi.org/10.1016/j.csda.2019.106895 COMSTA 106895

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Computational Statistics and Data Analysis

Received date : 28 November 2018 Revised date : 28 November 2019 Accepted date : 28 November 2019 Please cite this article as: M. Hušková, S.G. Meintanis and C. Pretorius, Tests for validity of the semiparametric heteroskedastic transformation model. Computational Statistics and Data Analysis (2019), doi: https://doi.org/10.1016/j.csda.2019.106895. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2019 Elsevier B.V. All rights reserved.

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Tests for validity of the semiparametric heteroskedastic transformation model Marie Huškováa , Simos G. Meintanisb,c,∗, Charl Pretoriusa,c

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Department of Statistics, Charles University, Prague, Czech Republic Department of Economics, National & Kapodistrian University of Athens, Athens, Greece c Unit for Business Mathematics and Informatics, North-West University, Potchefstroom, South Africa

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b

Abstract

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There exist a number of tests for assessing the nonparametric heteroskedastic location-scale assumption. The goodness-of-fit tests considered are for the more general hypothesis of the validity of this model under a parametric functional transformation on the response variable, specifically testing for independence between the regressors and the errors in a model where the transformed response is just a location/scale shift of the error is considered. The proposed criteria use the familiar factorisation property of the joint characteristic function of the covariates under independence. The difficulty is that the errors are unobserved and hence one needs to employ properly estimated residuals in their place. The limit distribution of the test statistics is studied under the null hypothesis as well as under alternatives, and also a resampling procedure is suggested in order to approximate the critical values of the tests. This resampling is subsequently employed in a series of Monte Carlo experiments that illustrate the finite-sample properties of the new test. The performance of related test statistics for normality and symmetry of errors is also investigated, and application of our methods on real data sets is provided.

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Keywords: Bootstrap test, heteroskedastic transformation, independence model, nonparametric regression

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Corresponding author, 1 Sophocleous Street 105 59 Athens, Greece (On sabbatical leave from the University of Athens) Email address: [email protected] (Simos G. Meintanis) ∗

Preprint submitted to Elsevier

November 28, 2019

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1. Introduction

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At least since the seminal paper of Box and Cox (1964) transformations are applied to data sets in order to facilitate statistical inference. The goal of a certain transformation could be, among others, reduction of skewness, faster convergence to normality and better fit, linearity, and stabilisation of variance. These aims, which may even be contradictory, are quite important as it is only under such assumptions that certain statistical procedures are applicable. Some of the issues raised when performing a certain transformation are discussed in varying settings by Staniswalis et al. (1993), Quiroz et al. (1996), Yeo and Johnson (2000), Chen et al. (2002), Mu and He (2007), and Meintanis and Stupfler (2015), and in the reviews of Sakia (1992) and Horowitz (2009), chapter 6. Here we will consider goodness-of-fit (GOF) tests for the (after-transformation) location-scale nonparametric heteroskedastic model T (Y ) = m(X) + σ(X)ε,

(1)

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where T (·) is a transformation acting on the response Y , m(·) and σ(·) are unknown functions, and where the error ε, having mean zero and unit variance, is supposed to be independent of the vector of covariates X. The classical location-scale model, i.e. the model in (1) with T (Y ) ≡ Y , is a popular model that is often employed in statistics as well as in econometrics (see e.g., Racine and Li, 2017; Brown and Levine, 2007; Chen et al., 2005), and there exist a number of approaches to test the validity of this model such as the classical Kolmogorov–Smirnov and Cramér–von Mises tests in Einmahl and Van Keilegom (2008) and the test criteria in Hlávka et al. (2011) which are based on the characteristic function. A related problem is to test for a specific family of regression functions; see for instance Van Keilegom et al. (2008). For an up-to-date review article on the subject of model testing in the context of regression (without transformation) we refer the interested reader to González-Manteiga and Crujeiras (2013). On the other hand the problem of goodness-of-fit (GOF) for the general model (1) under any given (fixed) transformation has only recently drawn attention in Neumeyer et al. (2016) by means of classical methods. Here we deviate from classical approaches in that we employ the characteristic function (CF) instead of the distribution function (DF) as the basic inferential tool. As already mentioned, the CF approach was also followed in analogous situations by Stute and Zhu (2005), Hlávka et al. (2011), and Hušková et al. (2018), among others, and gave favorable results. The rest of the paper is outlined as follows. In Section 2 we introduce the null hypothesis of independence between the regressor and the error term and formulate the new test statistic. The asymptotic distribution of the test statistic under the null 2

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2. Null hypothesis and test statistics

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hypothesis as well as under alternatives is studied in Section 3, while in Section 4 we particularize our method, and suggest a bootstrap procedure for its calibration. Section 5 presents the results of a Monte Carlo study. Since one aim of transforming the response is to achieve normality, or more generally symmetry after-transformation, we also investigate the small-sample performance of CF-based statistics for these problems with reference to the regression errors. Real data applications are included in Section 6. We finally conclude with discussion of our findings in Section 7. Some technical material is deferred to the Appendix. Some additional simulation results are contained in an electronic supplement accompanying this paper.

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Note that underlying equation (1) is the potentiality of obtaining a location/scale structure following a certain transformation of the response. Hence there exist a number of inferential problems similar to the problems faced in the non-transformation case, i.e., when T (Y ) ≡ Y . For the homoskedastic version of model (1) with σ ≡ constant, estimation methods were proposed by Linton et al. (2008) and Colling et al. (2015). The classical problem of fitting a specific regression function was considered by Colling and Van Keilegom (2016, 2017), respectively, by means of the DF and the integrated regression function, while regressor significance using Bierens’ CF-type approach was considered by Allison et al. (2018). On the other hand, the problem of GOF of the model itself is studied by Hušková et al. (2018) only in the homoskedastic case, whereas the single existing work for GOF testing with the more general heteroskedastic model is that of Neumeyer et al. (2016) which is based on the classical pair of Kolmogorov–Smirnov/Cramér–von Mises functionals. Here we are concerned with the GOF test for a fixed parametric transformation Yϑ = Tϑ (Y ), indexed by a parameter ϑ ∈ Θ. Specifically on the basis of independent copies (Yj , Xj ), j = 1, . . . , n, of (Y, X) ∈ R × Rp we wish to test the null hypothesis

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H0 : ∃ ϑ0 ∈ Θ such that εϑ0 (Y, X)⊥X,

(2)

where ⊥ denotes stochastic independence, and εϑ (Y, X) =

Yϑ − mϑ (X) , σϑ (X)

(3)

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with mϑ (X) := E(Yϑ |X) and σϑ2 (X) := Var(Yϑ |X) being the mean and variance, respectively, of the transformed response conditionally on the covariate vector X, and Θ ⊆ Rq , q ≥ 1. Note that the validity of the null hypothesis H0 is tantamount to the existence of a (true) value ϑ0 of ϑ which if substituted in this specific parametric 3

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where

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transformation and applied on the response will render the location/scale structure of model (1). To avoid confusion we emphasize that while the transformation is indeed parametric, the regression and the heteroskedasticity functions, respectively mϑ (·) and σϑ (·), and apart from their implicit dependence on the particular transformation Tϑ (·) and the associated parameter ϑ, they are both viewed and estimated within an entirely nonparametric context. Therefore our model can be labelled as a semiparametric model, i.e. parametric in the transformation but nonparametric in its location and scale functions. To motivate our test statistic write ϕX,εϑ for the joint CF of (X, εϑ ), and ϕX and ϕεϑ for the marginal CFs of X and εϑ , respectively, and recall that the null hypothesis in (2) equivalently implies that ϕX,εϑ0 = ϕX ϕεϑ0 , so that by following the approach in Hušková et al. (2018), the suggested test procedure will be based on the criterion Z ∞Z |ϕ(t b 1 , t2 ) − ϕ bX (t2 )ϕ bεb(t1 )|2 W (t1 , t2 )dt1 dt2 , (4) ∆n,W = n n

1X exp{it> bj } ϕ(t b 1 , t2 ) = 2 Xj + it1 ε n j=1

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is the joint empirical CF, and ϕ bX (t2 ) and ϕ bεb(t1 ) are the empirical marginal CFs resulting from ϕ(t b 1 , t2 ) by setting t1 = 0 (resp. t2 = 0). These three quantities serve as estimators of ϕX,εϑ0 , ϕX and ϕεϑ0 , respectively, and they will be computed by means of properly estimated residuals εbj = (Ybj − m(X b j ))/b σ (Xj ), j = 1, . . . , n, where m(·) b and σ b(·) are estimators of mϑ0 (·) and σϑ0 (·) discussed in the next section. We note that in these residuals the observed response is parametrically transformed by means of Ybj = Tϑb(Yj ), using the particular transformation under test and the b of the transformation parameter ϑ. Other than that and corresponding estimate ϑ as already mentioned, the estimate m(·) b of the regression function as well as the heteroskedasticity estimate σ b(·) are obtained entirely nonparametrically. Having said b in these estimates. Clearly for any weight function this, we often suppress the index ϑ W satisfying W (·, ·) > 0, the test statistic ∆n,W defined in (4) is expected to be large under alternatives, and therefore large values indicate that the null hypothesis is violated. We note that our formulation (4) for the test statistic is quite general with respect to the weight function W (t1 , t2 ). Nevertheless, and as it will be seen further down the paper (refer to Section 4.1 and the Appendix), we impose certain mild conditions on the weight function, the weakest one being that this function is integrable. 4

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An alternative approach is to apply a specific non-integrable weight function proportional to (|t1 |2 kt2 k1+p )−1 . If so, then we will end up with the empirical distance covariance as test statistic that was first put forward by Székely et al. (2007), and later investigated by many others; see the recent review by Edelmann et al. (2019). For further discussion and interrelations between our approach and that of Székely et al. (2007) the reader is referrred to Chen et al. (2019). 3. Theoretical results

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We now consider theoretical properties of the introduced test statistics. More precisely, we present the limit distribution of our test statistics under both the null as well as alternative hypotheses. Since the assumptions are quite technical they are deferred to the Appendix. We first introduce some notation. For ϑ ∈ Θ, define

mϑ (Xj ) = E Tϑ (Yj )|Xj and σϑ2 (Xj ) = Var Tϑ (Yj )|Xj . (5)

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As estimators one can use local polynomial estimators, the most widely used being local constant and local linear estimators. In order to avoid too many technicalities, the theoretical results of our test procedure are formulated and proved only for local constant regression estimators (classical kernel type estimators). However, theoretical results for local linear estimators can be formulated and proved in a similar way under slightly different assumptions with more technical proofs (see, e.g., Linton et al. (2008) and Neumeyer et al. (2016) for similar setups). The locally constant kernel estimators of mϑ (x) and σϑ2 (x), x = (x1 , . . . , xp )> , are defined by n 1 1 X  x − Xv  K Tϑ (Yv ), m b ϑ (x) = h fb(x) nhp v=1 n 2 1 1 X  x − Xv  σ bϑ2 (x) = K T (Y ) − m b (X ) , ϑ v ϑ v h fb(x) nhp v=1

respectively, where K(·) and h = hn are a kernel and a bandwidth, and n 1 X  x − Xv  b f (x) = K nhp v=1 h

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is a kernel estimator of the density of Xj . Finally, let εϑ,j =

Tϑ (Yj ) − mϑ (Xj ) , σϑ (Xj )

εj = εϑ0 ,j , 5

εbj = εbϑ,j b =

Tϑb(Yj ) − m b ϑb(Xj ) , σ bϑb(Xj )

(6)

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b is a √n-consistent estimator of ϑ0 . It is assumed that ϑ b allows an asymptotic where ϑ representation specified by assumption (A.7), which include estimators based on the mean square distance from independence (see, e.g., Linton et al., 2008) and estimators based on a profile likelihood approach (see Neumeyer et al., 2016). Now we formulate the limit distribution of the test statistic under the null hypothesis:

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Theorem 1. Let assumptions (A.1)–(A.8) be satisfied. Then under the null hypothesis, as n → ∞, Z d ∆n,W → |Z(t1 , t2 )|2 W (t1 , t2 )dt1 dt2 , Rp+1

where {Z(t1 , t2 ), (t1 , t2 ) ∈ Rp+1 } is a Gaussian process with zero mean function and the same covariance structure as the process {Z0 (t1 , t2 ), (t1 , t2 ) ∈ Rp+1 } defined as

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> Z0 (t1 , t2 ) = {cos(t1 ε1 ) − Cε (t1 )}g+ (t> 2 X1 ) + {sin(t1 ε1 ) − Sε (t1 )}g− (t2 X1 )   > + t1 ε1 Sε (t1 )g+ (t> 2 X1 ) + Cε (t1 )g− (t2 X1 )   1 0 > + t1 (ε21 − 1) Cε0 (t1 )g+ (t> X ) − S (t )g (t X ) 1 1 − 1 2 ε 2 2 > + g (Y1 , X1 )Hϑ0 ,q (t1 , t2 ),

(7)

where Cε and Sε are the real and the imaginary part of the CF of ε1 . Similarly, CX and SX denote the real and the imaginary part of the CF of Xj . Also, g(Y1 , X1 ) is specified in Assumption (A.7) and

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> > g+ (t> 2 X1 ) = cos(t2 X1 ) + sin(t2 X1 ) − CX (t2 ) − SX (t2 ),

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> > g− (t> 2 X1 ) = cos(t2 X1 ) − sin(t2 X1 ) − CX (t2 ) + SX (t2 ), h ∂T (Y ) ∂Tϑ (Y1 ) > ϑ 1 ,..., Hϑ,q (t1 , t2 ) = E ∂ϑ1 ∂ϑq n 1   > × − t1 sin(t1 ε1 )g+ (t> X ) + t cos(t ε )g (t X ) 1 1 1 1 − 2 1 2 σ(X1 )  oi 1 > + (1 + ε1 ε2 ) t1 sin(t1 ε2 )g+ (t> X ) − t cos(t ε )g (t X ) . 2 1 1 2 − 2 2 2 σ(X2 )

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The proof is postponed to the Appendix. The limit distribution under the null hypothesis is a weighted L2 -type functional of a Gaussian process. Concerning the structure of Z0 (·, ·), the first row in (7) corresponds to the situation when ϑ0 , ε, m(·), σ 2 (·) are known, the second row 6

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reflects the influence of the estimator of m(·), the third one of the estimator of σ 2 (·) while the last row reflects the influence of the estimator of ϑ. To get an approximation of the critical value one estimates the unknown quantities and simulates the limit distribution described above with unknown parameters replaced by their estimators. However, the bootstrap described in Section 4.2 is probably more useful. Concerning the consistency of the newly proposed test, note that if H0 is not true, then there is no parameter in ϑ that leads to independence. The main assertion under alternatives reads as follows. b converges in probability to some ϑ0 ∈ Θ Theorem 2. Assume that the estimator ϑ and let ϑ0 ∈ Θ satisfy Z |ϕX,εϑ0 (t2 , t1 ) − ϕX (t2 )ϕεϑ0 (t1 )|2 W (t1 , t2 )dt1 dt2 > 0. (8) Rp

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Let assumptions (A.1)–(A.4), (A.8) and (A.9) be satisfied and let also (A.5), (A.6) with s1 = s2 = 1 and ϑ0 replaced by ϑ0 . Then as n → ∞, P

∆n,W → ∞.

The proof is deferred to the Appendix.

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Remark 1. When the null hypothesis H0 is not satisfied, the squared modulus figuring in (8) will be non-zero and thus the left-hand side of (8) will indeed be strictly positive, provided that W > 0, regardless of the limiting value ϑ0 . Consequently a test that rejects the null hypothesis H0 for large values of ∆n,W will have power equal to one asymptotically.

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4. Computations and resampling

4.1. Computations Following Hušková et al. (2018), we impose the decomposition W (t1 , t2 ) = w1 (t1 )w2 (t2 ) on the weight function. If in addition the individual weight functions wm (·), m = 1, 2, satisfy all other requirements stated in Assumption (A.8) of the Appendix, then the test statistic in (4) takes the form n n n n X 1 X 2 X 1 X I1,jk I2,jk + 3 I1,jk I2,jk − 2 I1,jk I2,j` , n j,k=1 n j,k=1 n j,k=1 j,k,`=1

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(9)

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where I1,jk := Iw1 (b εjk ), I2,jk := Iw2 (Xjk ), with Xjk = Xj − Xk and εbjk = εbj − εbk , j, k = 1, . . . , n, and Z Iwm (x) = cos(t> x)wm (t)dt, m = 1, 2. (10)

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The weight function wm (·) in (10) may be chosen in a way that facilitates integration which is extremely important in high dimension. To this end notice that if wm (·) is replaced by a spherical density, then the right-hand side of (10) gives (by definition) the CF corresponding to wm (·) computed at the argument x. Furthermore, recall that within the class of all spherical distributions, the integral in (10) depends on x only via its usual Euclidean norm kxk, and specifically Iwm (x) = Ψ(kxk), where the functional form of the univariate function Ψ(·) depends on the underlying subfamily of spherical distributions. In this connection Ψ(·) is called the “characteristic kernel” of the particular subfamily; see Fang et al. (1990). Consequently the test statistic in (9) becomes a function of Ψ(kxk) alone. Subfamilies of spherical distributions with simple kernels are the class of scaled spherical stable distributions with γ (S) Ψγ (u) = e−cu , 0 < γ ≤ 2, and the class of generalized multivariate Laplace distri(L) butions with Ψγ (u) = (1 + (u2 /c))−γ , γ > 0, for any c > 0. For more information on these particular cases the reader is referred to Nolan (2013), and to Kozubowski et al. (2013), respectively. For further use we simply note that interesting special cases of spherical stable distributions are the Cauchy distribution and the normal (S) distribution corresponding to Ψγ with γ = 1 and γ = 2, respectively, while the (L) classical multivariate Laplace distribution results from Ψγ for γ = 1.

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4.2. Resampling Recall that the null hypothesis H0 in (2) corresponds to model (1) in which both the true value of transformation parameter ϑ as well as the error density are unknown. In this connection, and since, as was noted in Section 2, the asymptotic distribution of the test criterion under the null hypothesis depends on these quantities, among other things, we provide here a resampling scheme which can be used in order to compute critical points and actually carry out the test. The resampling scheme, which was proposed by Neumeyer et al. (2016), involves resampling from the observed Xj and independently constructing the bootstrap errors by smoothing the residuals. The bootstrap model then fulfils the null hypothesis since
ε∗j Tϑb(Yj∗ ) − E∗ Tϑb(Yj∗ ) Xj∗ q p ⊥∗ Xj∗ , :=
2 ∗ ∗ ∗ 1 + a n Var Tϑb(Yj ) Xj 8

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where E∗ and Var∗ denote the conditional expectation and variance and ⊥∗ the conditional independence given the original sample, with an > 0 being a smoothing parameter. We now describe the resampling procedure. Let the smoothing parameter an be such that an → 0 and nan → ∞, as n → ∞. Also, denote by {ξj }nj=1 a sequence of random variables which are drawn independently of any other stochastic quantity involved in the test criterion. The bootstrap procedure is as follows:

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1. Draw X1∗ , . . . , Xn∗ with replacement from X1 , . . . , Xn .

2. Generate i.i.d. random variables {ξj }nj=1 with a standard normal distribution and let ε∗j = an ξj + εbj , j = 1, ..., n, with εbj defined in (6).

3. Compute the bootstrap responses Yj∗ = Tϑb−1 (m bϑb(Xj∗ )ε∗j ), j = b ϑb(Xj∗ ) + σ 1, . . . , n.

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4. On the basis of the observations (Yj∗ , Xj∗ ), j = 1, . . . , n, refit the model and obtain the bootstrap residuals εb∗j , j = 1, . . . , n.

5. Calculate the value of the test statistic, say ∆∗n,W , corresponding to the bootstrap sample (Yj∗ , Xj∗ ), j = 1, . . . , n. ∗(b)

6. Repeat the previous steps a number of times, say B, and obtain {∆n,W }B b=1 . 7. Calculate the critical point of a size-α test as the (1 − α) level quantile c∗1−α of ∗(b) ∆n,W , b = 1, ..., B.

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8. Reject the null hypothesis if ∆n,W > c∗1−α , where ∆n,W is the value of the test statistic based on the original observations (Yj , Xj ), j = 1, . . . , n.

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b which It may be shown that this bootstrap scheme, when based on an estimate ϑ will be further specified in the next section, consistently estimates the asymptotic null distribution of the test statistic in the sense that lim |P(∆∗n,W ≤ x) − P(∆n,W ≤ x)| = 0, ∀x.

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For more details on the consistency of the smooth bootstrap the reader is referred to Neumeyer (2009).

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5. Simulations

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In this section we present the results of a Monte Carlo exercise that sheds light on the small-sample properties of the new test statistic and compare our test with the classical Kolmogorov–Smirnov (later denoted by KS) and Cramér–von Mises (CM) criteria suggested by Neumeyer et al. (2016). We considered the family of transformations   (Y + 1)ϑ − 1 /ϑ if Y ≥ 0, ϑ 6= 0,     log(Y + 1) if Y ≥ 0, ϑ = 0,  Tϑ (Y ) = 2−ϑ  − (−Y + 1) − 1 /(2 − ϑ) if Y < 0, ϑ 6= 2,    − log(−Y + 1) if Y < 0, ϑ = 2,

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proposed by Yeo and Johnson (2000), and randomly generated paired observations (Yj , Xj ), j = 1, . . . , n, from the univariate heteroskedastic model Tϑ0 (Y ) = m(X) + σ(X)ε,

(11)

where ϑ0 = 0, m(x) = 1.5 + exp(x) and σ(x) = x. Here, ε is an error term which should be stochastically independent of X under the null hypothesis. The distribution of the covariate X and the distribution of the error ε (conditional on X) were chosen as one of the following: Model A. X ∼ uniform(0, 1). Let ST(ζ, ω, η, ν) denote the univariate skew-t distribution with parameters ζ (location), ω (scale), η (shape) and ν (degrees of freedom) as defined by Azzalini (2005). Define

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( Wη,ν −E(Wη,ν ) √ Var(Wη,ν )

Z

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d (ε X = x) =

if 0 ≤ x ≤ 0.5, if 0.5 < x ≤ 1,

where Wη,ν ∼ ST(0, 1, η, ν) and Z ∼ N(0, 1), both quantities independent of X. Notice that the null hypothesis of a heteroskedastic transformation structure is violated except when η → 0 and ν → ∞, in which case Wη,ν → N(0, 1) so that ε and X are independent.

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Model B. X ∼ uniform(0, 1). Define d (ε X = x) =

(

W √ν −ν 2ν

Z

10

if 0 ≤ x ≤ 0.5, if 0.5 < x ≤ 1,

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where Wν ∼ χ2ν and Z ∼ N(0, 1). Notice that ε is stochastically dependent on X except when ν → ∞, in which case the null hypothesis of a heteroskedastic transformation structure is satisfied.

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Model C. X ∼ uniform(0, 1). Let AL(ν, λ, κ) denote the univariate asymmetric Laplace distribution with parameters ν (location), λ (scale) and κ (shape) as studied by Kozubowski et al. (2013). Define   W√κ −(1−κ2 )/κ if 0 ≤ x ≤ 0.5, d (1+κ4 )/κ2 (ε X = x) = Z if 0.5 < x ≤ 1,

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where Wκ ∼ AL(0, 1, κ) and Z ∼ AL(0, 1, 1), the latter being an observation from the usual symmetric Laplace distribution. Notice that ε is stochastically dependent on X except when κ = 1, in which case the null hypothesis is satisfied.

Model D. To investigate the behavior of the tests also in the case of a discrete covariate, we considered generating X from a discrete uniform distribution 1 2 , 10 , · · · , 1}, with ε having the same distribution as the errors on the set { 10 of Model A given above.

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For a test size of α = 0.05, the rejection frequency of the test was recorded for sample sizes n = 100, 200, 300. The bootstrap resampling scheme of Section 4.2 requires a choice of the smoothing parameter an , where we followed Neumeyer et al. (2016) and chose an = 0.5n−1/4 throughout. Since the bootstrap replications are time consuming we have employed the warp-speed method of Giacomini et al. (2013) in order to calculate critical points of the test criterion. With this method we generate only one bootstrap resample for each Monte Carlo sample and thereby compute the bootstrap test statistic ∆∗n,W for that resample. Then, for a number M of Monte Carlo replications, the size-α critical point is determined similarly as in step 7 of ∗(m) Section 4.2, by computing the (1 − α)-level quantile of ∆n,W , m = 1, ..., M . For all simulations the number of Monte Carlo replications was set to M = 5 000. To estimate the transformation parameter ϑ0 in (11) we employ the profile likelihood estimator recently studied by Neumeyer et al. (2016). The resulting estimator, say ϑbPL , allows for the heteroskedastic error structure present in our setup. Implementation of this estimator involves some practical considerations, which we now discuss. The estimator ϑbPL requires estimation of m(·) and σ(·) nonparametrically, for which we used local linear regression with a Gaussian kernel with bandwidth chosen using the direct plug-in methodology proposed by Ruppert et al. (1995). 11

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Specifically we use b hDPI defined in step 3 on p. 1263 of Ruppert et al. (1995), where in step 1 of their procedure we also take N ∗ = 5. Secondly, the estimator ϑbPL requires estimation of the density f (·) of the regression errors. For this purpose we used a Gaussian kernel with a two-stage plug-in bandwidth chosen as described on p. 72 of Wand and Jones (1994) (also see Sheather and Jones, 1991). We have also tried local constant regression for m(·) and σ(·). Corresponding results showing some moderate size distortion are provided in the electronic supplement accompanying this paper. Our test also requires estimation of the unknown functions m(·) and σ(·) figuring in (5). To be consistent, we use the same local linear regression estimators for these functions as is used in the calculation of ϑbPL described in the previous paragraph. Both methods of bandwidth selection described above have been implemented by Wand (2015) and are readily available as part of the R package KernSmooth. The author warns that in some cases these procedures might be numerically unstable (see p. 10 of Wand, 2015). In these rare situations we turned to simple rule of thumb selection methods: in the case of nonparametric regression we used the rule of thumb of Fan and Gijbels (1996, p. 111) (implemented in the package locpol by Cabrera, 2018) and in the case of density estimation a rule of Silverman (1986, equations (3.28) and (3.30) on pp. 45 and 47). Finally, to actually implement nonparametric regression and density estimation using these chosen bandwidths, we employed the package np (Hayfield and Racine, 2008) designed specifically for this purpose.

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5.1. Simulation results for independence The simulation results for the four considered models are shown in Tables 1 to 4. The results for the classical Kolmogorov–Smirnov and Cramér–von Mises tests are given in the columns labelled KS and CM, respectively. The percentage of rejections of our statistic ∆n,W is given for three different choices of the characteristic kernel Ψ(·) discussed Section 4.1, with various choices of the tuning parameter c > 0. Specifically and as already mentioned, we use as weight functions scaled spherical γ (S) stable and Laplace densities that yield Ψγ = e−cu (recall that γ = 2 coincides (L) with the Gaussian case) and Ψγ = (1 + (u2 /c))−γ , respectively. From Tables 1 to 4 it is clear that the test based on ∆n,W respects the nominal size well. However, for smaller sample sizes the test appears to be slightly conservative in some cases. The same can be said of the classical KS and CM tests. Notice that, under the various considered alternatives, the power of all tests increase in accordance with the nature of the dependence that is introduced between the covariates and the error terms. Moreover, in agreement with the consistency of the test established formally in Theorem 2, under alternatives the power of our test appears to increase as the sample size increases. Overall, in terms of power the new test based on ∆n,W exhibits 12

Journal Pre-proof

Table 1. Size and power results for verifying the validity of Model A. The null hypothesis is satisfied for η = 0 and ν = ∞. The nominal size of the test is α = 0.05. ∆n,W =

(S) Ψ1 (u)

exp(−cu2 )

η

ν

n

KS

CM

c=1

0

∞

100 200 300

4.7 4.6 5.0

3.9 4.2 4.9

4.5 4.6 5.8

4.4 4.9 5.9

4.4 4.8 6.1

4.2 4.0 5.6

3.8 3.9 4.9

0

5

100 200 300

7.1 9.3 11.4 15.5 15.7 21.5

10.4 18.3 26.5

9.9 18.7 27.9

9.4 18.6 27.5

8.7 17.3 25.0

8.9 16.5 24.6

8.7 17.7 24.5

8.5 17.0 24.3

2.1 100 22.3 24.5 200 41.0 44.2 300 56.4 60.5

31.2 51.5 69.3

31.4 52.9 70.3

31.2 53.4 71.7

31.2 54.2 71.4

29.4 52.0 69.2

30.3 53.4 70.7

2.1

39.8 57.5 67.1

40.2 58.1 68.5

40.4 58.2 69.6

41.8 58.9 71.8

42.0 59.6 71.1

43.2 60.9 72.0

100 33.0 35.8 200 51.3 52.2 300 67.1 66.8

c = 0.5 c = 1 4.0 3.7 5.0

c = 1.5 c = 2 3.7 3.8 4.9

c = 0.1 c = 0.25 c = 0.5

3.6 3.7 4.8

4.2 4.4 5.6

4.3 4.6 5.7

4.3 4.9 5.6

8.0 15.9 23.9

7.6 14.4 22.5

8.6 17.5 25.6

9.2 18.1 27.2

10.2 18.4 26.3

30.5 54.7 71.0

30.2 54.1 71.4

29.4 53.6 70.4

31.5 54.2 71.5

31.8 53.6 71.2

31.6 52.3 70.0

44.3 61.3 73.3

44.2 61.7 74.7

44.2 61.0 75.0

42.5 59.9 71.9

41.1 58.9 70.2

40.4 57.5 68.2

Table 2. Size and power results for verifying the validity of Model B. The null hypothesis is satisfied for ν = ∞. The nominal size of the test is α = 0.05. ∆n,W

(S) Ψ2 (u)

(S) Ψ1 (u)

= exp(−cu2 )

(L)

Ψ1 (u) = (1 + u2 /c)−1

= exp(−cu)

KS

CM

c=1

5.1 3.9 4.7

3.8 4.1 4.7

5.1 4.3 5.9

4.9 4.2 5.5

4.6 4.1 5.3

4.4 4.0 5.1

4.4 3.7 4.8

4.5 3.6 4.8

4.4 3.5 4.5

4.1 3.4 4.6

4.7 4.0 5.3

5.0 4.4 5.5

5.0 4.3 5.7

4.9 4.2 5.8

10

100 5.9 7.1 200 9.7 10.2 300 10.0 11.0

8.9 13.5 14.4

8.4 12.7 13.4

7.9 12.2 12.6

6.8 11.0 11.4

7.7 11.6 12.9

7.2 11.4 12.9

6.9 10.9 12.4

6.6 10.3 12.0

8.1 12.2 13.2

8.5 13.0 14.0

9.1 13.1 14.3

9.5 13.6 14.7

5

100 9.7 10.4 200 16.1 17.6 300 14.7 18.1

13.3 21.6 23.7

12.0 20.8 23.0

11.4 20.5 22.5

11.2 19.7 22.1

12.2 21.6 23.6

12.2 22.1 23.4

11.9 20.9 23.0

11.4 20.6 22.9

11.8 21.4 23.1

12.8 22.0 23.6

13.3 22.5 23.9

14.0 21.9 24.1

3

100 14.4 17.4 200 22.5 26.1 300 31.6 35.4

22.7 30.6 36.6

21.5 30.6 36.6

21.1 30.4 37.3

18.7 30.6 39.2

21.1 32.3 38.8

21.1 32.6 39.5

20.2 32.5 39.6

19.0 32.5 40.4

21.5 31.5 38.0

22.2 31.7 37.1

22.3 31.7 36.7

22.5 31.0 36.5

2

100 21.0 25.2 200 39.1 43.0 300 53.0 52.2

30.2 47.0 53.8

29.6 47.9 54.9

28.7 48.1 56.3

28.3 50.7 59.4

29.6 49.9 57.6

30.3 51.5 59.3

30.0 52.4 60.4

29.7 52.7 61.3

29.8 49.5 57.4

29.9 47.9 54.6

29.9 46.9 53.7

29.2 46.2 52.6

n

Jo

urn

∞ 100 200 300

c = 1.5 c = 2

c=4

c = 0.5 c = 1

al

ν

13

c=1

3.9 3.5 5.0

p ro

100

c=4

Pr e-

0

c = 1.5 c = 2

(L)

Ψ1 (u) = (1 + u2 /c)−1

= exp(−cu)

of

(S) Ψ2 (u)

c = 1.5 c = 2

c = 0.1 c = 0.25 c = 0.5

c=1

Journal Pre-proof

∆n,W (S) Ψ2 (u)

=

(S) Ψ1 (u)

exp(−cu2 )

of

Table 3. Size and power results for verifying the validity of Model C. The null hypothesis is satisfied for κ = 1. The nominal size of the test is α = 0.05.

(L)

Ψ1 (u) = (1 + u2 /c)−1

= exp(−cu)

κ

n

KS

CM

c=1

c = 1.5

c=2

c=4

c=1

1

100 200 300

5.1 5.9 6.5

4.8 6.2 5.5

5.0 5.2 5.8

5.5 5.3 5.8

5.4 5.5 5.4

5.2 5.0 5.5

5.0 5.2 4.9

4.8 4.9 5.2

4.7 4.9 5.2

2

100 8.6 200 20.7 300 34.0

9.4 23.8 32.3

13.9 30.2 33.3

14.1 30.5 35.1

13.9 30.7 35.7

13.9 31.9 41.0

13.6 29.9 37.2

14.0 31.4 39.3

5

100 10.7 12.1 200 27.8 33.4 300 41.3 41.1

18.3 38.7 43.6

18.6 40.3 45.7

19.2 41.7 47.0

19.8 44.2 57.0

18.6 42.3 53.5

19.2 43.3 56.0

c = 1.5 c = 2

c=4

c = 0.01 c = 0.025 c = 0.05 c = 0.1 4.0 4.3 5.4

4.8 4.7 5.6

5.0 4.7 5.0

4.6 4.9 4.9

13.5 31.1 38.6

11.5 30.8 44.3

10.0 24.9 39.8

11.1 30.1 43.6

11.9 30.8 44.1

13.3 32.0 40.3

20.2 44.7 59.2

19.6 45.6 66.7

16.1 41.7 63.6

17.6 44.5 65.8

19.4 46.5 66.9

20.5 45.9 64.1

Pr e-

p ro

4.7 4.6 5.3

Table 4. Size and power results for verifying the validity of Model D. The null hypothesis is satisfied for η = 0 and ν = ∞. The nominal size of the test is α = 0.05. ∆n,W

(S) Ψ2 (u)

(S) Ψ1 (u)

= exp(−cu2 )

(L)

Ψ1 (u) = (1 + u2 /c)−1

= exp(−cu)

ν

n

KS

CM

c=1

0

∞

100 200 300

3.1 3.7 4.5

2.8 3.5 4.0

3.9 4.4 4.6

3.5 4.6 4.8

3.3 4.3 4.6

3.8 4.0 4.8

3.6 4.2 4.8

3.5 4.5 4.5

3.5 4.7 4.5

3.4 4.2 4.6

3.7 3.9 4.8

3.7 4.1 4.9

3.5 4.4 4.5

3.4 4.5 4.4

0

5

100 200 300

5.7 7.2 9.7

5.8 8.6 11.2

7.4 9.8 11.1

7.6 10.0 12.1

7.1 10.4 13.2

6.8 10.4 13.3

7.1 10.8 13.1

7.1 11.1 13.7

7.3 11.0 14.0

6.8 10.6 13.3

6.3 10.6 12.9

6.9 10.7 13.6

7.0 11.0 13.7

7.3 10.5 13.8

2.1 100 18.9 19.0 200 35.3 33.3 300 53.4 49.1

22.1 39.4 56.1

23.0 42.7 60.4

25.3 46.7 64.9

26.2 48.6 67.5

25.5 46.6 64.4

25.8 48.8 66.6

26.1 49.9 68.1

27.2 50.9 67.9

25.7 48.3 68.0

26.6 49.7 68.0

26.6 49.8 67.4

25.7 47.8 65.7

2.1

32.1 54.2 64.8

34.5 57.5 68.2

36.0 61.5 72.6

37.7 63.9 75.8

38.2 60.3 70.8

38.2 62.3 72.1

38.3 63.7 73.5

40.1 65.8 76.8

38.8 66.8 77.6

39.4 65.6 76.5

38.4 63.5 74.3

36.6 61.6 72.2

c = 1.5 c = 2

urn

100

100 30.1 27.7 200 53.9 53.1 300 69.1 65.2

Jo

0

al

η

c=4

14

c = 0.5 c = 1

c = 1.5 c = 2

c = 0.1 c = .25 c = .5 c = 1

Journal Pre-proof

Table 5. Size and power results for verifying the validity of Model A using the classical nonsmooth residual bootstrap. The null hypothesis is satisfied for η = 0 and ν = ∞. The nominal size of the test is α = 0.05. ∆n,W (S) Ψ1 (u)

= exp(−cu2 )

η

ν

n

KS

CM

c=1

0

∞

100 200 300

2.1 2.7 2.4

1.4 2.5 2.2

2.3 3.7 3.3

2.4 3.6 3.2

2.1 3.6 3.0

2.2 3.1 2.6

1.4 2.4 2.1

1.5 2.3 2.0

0

5

100 200 300

4.2 7.0 7.1

5.3 8.4 10.1

9.4 12.4 15.6

9.0 12.4 15.7

9.3 11.9 15.2

8.2 10.9 12.4

6.8 9.4 12.3

2.1 100 16.1 18.3 200 29.4 30.9 300 40.3 40.7

27.0 41.4 51.8

28.5 43.3 53.7

28.6 43.1 54.6

27.4 43.2 54.2

2.1

33.6 52.9 61.6

33.5 53.6 62.8

33.4 54.3 63.4

32.1 54.7 64.1

100 20.8 22.4 200 43.6 45.2 300 55.3 56.1

c = 0.5 c = 1

c = 1.5 c = 2

c = 0.1 c = 0.25 c = 0.5

1.5 2.0 2.1

1.8 2.0 2.3

2.0 2.9 2.5

2.0 3.2 2.8

2.0 3.3 3.1

6.6 9.6 11.9

6.6 9.1 11.2

6.3 8.6 10.6

6.2 8.6 10.1

7.5 10.7 13.3

8.6 11.2 13.8

8.8 12.4 14.7

23.7 38.3 50.3

25.2 39.1 51.2

25.8 39.9 52.1

24.4 39.2 52.4

24.2 40.0 51.5

27.4 42.2 53.6

27.6 42.3 53.7

26.5 41.6 51.6

31.8 54.8 63.6

32.9 55.4 64.5

32.3 55.1 65.4

32.1 55.4 65.8

32.0 55.6 65.5

33.3 54.9 64.4

33.6 54.7 63.8

33.2 53.2 62.1

competitive performance and even outperforms the classical tests for most considered choices of the tuning parameter c. We close by noting that the value of the tuning parameter c clearly has some effect on the power of the test based on ∆n,W . There exist several interpretations regarding the value of c and for more information on this the reader is referred to the recent review paper by Meintanis (2016). For all tests we chose values of c for which the tests exhibit good size properties as well as good power under alternatives.

Jo

urn

al

Classical nonsmooth residual bootstrap The bootstrap resampling scheme presented in Section 4.2 involves smoothing the residuals to obtain bootstrap errors. Recently however, Neumeyer and Van Keilegom (2019) provided conditions under which the classical nonsmooth residual bootstrap is also asymptotically valid in the context of nonparametric regression. Their findings suggest that it is typically not necessary to smooth the residuals and, in the cases they considered, tests based on the nonsmooth residual bootstrap appeared to be more conservative and in that sense could be preferable to their counterparts based on the smoothed residual bootstrap. In other cases however, such as those with discrete regressors, the nonsmooth bootstrap shows extreme size distortions and therefore is not a viable option; see Racine and Van Keilegom (2019). To investigate the behavior of our test when the bootstrap errors are obtained simply by random sampling instead of smoothing the residuals, we provide some 15

c=1

1.5 2.2 2.0

p ro

100

c=4

Pr e-

0

c = 1.5 c = 2

(L)

Ψ1 (u) = (1 + u2 /c)−1

= exp(−cu)

of

(S) Ψ2 (u)

Journal Pre-proof

additional simulation results. Specifically, we consider a nonsmooth version of the resampling scheme set out in Section 4.2 in which we replace step 2 with the following:

of

2.0 Obtain the bootstrap errors {ε∗j }nj=1 by randomly sampling with replacement from the standardized residuals {b εj }nj=1 defined in (6).

Pr e-

p ro

Rejection percentages for Model A obtained by this nonsmooth resampling scheme are shown in Table 5. Comparing these results to the corresponding results in Table 1 shows that, under the nonsmooth residual bootstrap, all tests seem to be somewhat more conservative under the null hypothesis. This finding agrees with the observations made by Neumeyer and Van Keilegom (2019), although their results are for the nontransformation homoskedastic case (i.e. if in (1) one has T (Y ) = Y and σ(·) ≡ constant). Also, in terms of power the results seem to be in line with those seen in Table 1, with the added feature of power decrease which is expected however given the decrease in the empirical size. Results for Models B, C and D exhibit similar behavior and therefore are only shown in the Supplement.

urn

al

5.2. Simulation results for normality and symmetry One of the main goals of transformation is to reduce skewness and possibly even achieve near normality. These issues have been recently investigated by Yeo and Johnson (2000), Yeo et al. (2014), Meintanis and Stupfler (2015), and Chen et al. (2002), with or without regressors, with the last reference also providing asymptotics for a test of normality of the after Box–Cox transformation errors under homoskedasticity. In this section we investigate how the CF tests for symmetry and normality designed for i.i.d. data perform within the significantly more complicated context of the semiparametric heteroskedastic transformation model (1). In this connection we note that such CF tests have already shown competitive performance in more classical regression frameworks; see Hušková and Meintanis (2010, 2012). The CF test statistics of normality and symmetry are motivated by uniqueness of the CF of any given distribution, and by the fact that for any zero-symmetric distribution, the imaginary part of its CF is identically equal to zero. Thus we have the test statistic for normality Z ∆(G) n,w = n

∞

−∞

2 /2

|ϕ bεb(t) − e−t

|2 w(t)dt,

Jo

and the test statistic for symmetry of errors Z ∞ (S) ∆n,w = n (Im(ϕ bεb(t))2 w(t)dt, −∞

16

(12)

(13)

Journal Pre-proof

of

where w(·) is a weight function analogous to w1 (·) of assumption (A.8) and Im(z) denotes the imaginary part of a complex number z. In our Monte Carlo simulations, the results of which are shown in Tables 6 to 12, we use three choices of the weight function w(·) and various choices of the tuning parameter c for both tests. To obtain the critical value of the normality test corresponding to the statistic in (12), we used the same bootstrap resampling scheme as given in Section 4.2, but with step 2 replaced by:

p ro

2.00 Generate i.i.d. errors {ε∗j }nj=1 from a standard normal distribution.

The critical value of test for symmetry based on the statistic in (13) was obtained using a wild bootstrap scheme (see Neuhaus and Zhu, 2000; Delgado and GonzálezManteiga, 2001; Hušková and Meintanis, 2012) which is the same as that given in Section 4.2 but with step 2 replaced by:

Pr e-

2.000 Generate i.i.d. random variables {Uj }nj=1 according to the law P(Uj = +1) = P(Uj = −1) = 21 and set ε∗j = Uj ε˜j , j = 1, ..., n, where the ε˜j are drawn randomly with replacement from εb1 , . . . , εbn .

Jo

urn

al

Firstly, concerning the test for normality, we see from Tables 6 to 8 that the size of the test is approximately around the nominal size, being slightly conservative in some cases. The power of the test increases with the extend of violation of normality, i.e., as the skewness parameter η is increased or as the degrees of freedom parameter ν is decreased. Finally, the results seem to suggest consistency of the test in the sense that, for each given fixed alternative, the power increases gradually as the sample size is increased. These observations hold for error terms generated under Models A, B and D. For Model C, we just note that an analogous CF-based test can be constructed along the lines of Meintanis (2004). Regarding the test for symmetry similar conclusions as above can be made (see Tables 9 to 12). Note however that for Model A there is a clear over-rejection of the null hypothesis of symmetry in the cases where η = 0 and ν = 2.1 or 5, i.e. when the error distribution is symmetric but heavy-tailed. As seen in Table 11 this is also true for Model C in the case where κ = 1, although to a lesser extent. To address this issue we employed the permutation test suggested by Henze et al. (2003) developed specifically to address the issue of over-rejection. The results obtained in this way, however, agree almost exactly with the results in Tables 9 obtained using the wild bootstrap approach. It should be noted that this issue of over-rejection does not occur when the true transformation parameter ϑ0 is assumed to be known and only arises in the more complicated setting where ϑ0 needs to be estimated. 17

Journal Pre-proof

(G)

Table 6. Size and power results for assessing normality (∆n,w ) of the error terms appearing in Model A. The nominal size of the test is α = 0.05. w(t) = (1 + t2 /c2 )−1

w(t) = exp(−ct2 ) η

ν

n

c = 0.1

0

∞

100 200 300

3.7 3.7 3.9

4.0 3.8 4.0

4.2 3.8 4.1

4.4 4.0 4.6

2.5 3.1 2.8

2.6 3.0 2.9

2.6 2.9 3.0

5

∞

100 200 300

15.2 30.4 41.8

15.5 30.8 41.5

15.2 30.9 41.9

13.9 31.3 40.7

13.5 22.7 30.7

14.5 24.8 33.5

14.6 25.9 35.0

20

∞

100 200 300

18.5 43.0 55.1

19.8 42.2 54.1

19.5 41.4 52.1

18.8 41.4 50.2

14.9 27.3 37.4

15.9 30.0 41.3

0

5

100 200 300

5.5 6.3 6.1

6.0 6.7 6.5

6.4 7.1 6.8

6.8 7.8 7.5

7.2 9.4 13.7

2.1 100 200 300

14.0 27.3 29.0

14.7 25.4 27.7

14.6 24.3 26.2

14.1 23.4 26.2

2.1

36.1 57.6 58.1

34.1 53.9 53.6

32.6 51.7 52.9

29.9 50.5 53.1

100

100 200 300

c = 0.5 c = 1

w(t) = exp(−c|t|)

c = 1.5 c = 2

c = 0.05 c = 0.1 c = 0.25 c = 0.5 2.7 2.9 3.3

2.5 3.0 3.4

2.4 2.9 2.9

2.4 2.9 2.9

15.2 26.3 36.3

14.7 27.2 39.4

15.8 27.4 39.7

16.0 27.2 38.2

14.7 26.4 36.6

17.1 31.4 43.2

17.4 32.4 44.4

17.5 36.3 49.7

18.3 36.5 48.6

17.8 34.8 46.9

17.3 32.0 44.3

7.3 10.0 14.7

6.9 10.2 14.7

6.9 10.2 14.9

6.9 10.7 16.2

7.0 10.1 15.7

6.9 10.2 15.8

7.2 10.0 14.9

17.9 30.2 37.6

19.0 31.8 42.0

19.8 33.5 43.4

20.4 34.3 44.1

20.7 37.5 46.1

21.3 36.8 45.8

20.7 35.6 44.8

20.0 34.0 44.1

28.5 48.7 59.8

32.3 52.8 64.7

34.8 55.5 66.2

36.5 57.2 67.9

41.9 62.1 72.9

41.9 61.9 72.2

38.8 59.1 70.6

35.7 56.7 67.4

p ro

of

2.5 2.9 2.9

Pr e-

0

c = 0.25 c = 0.5 c = 1

al

In conclusion we note that the test for normality and the test for symmetry both exhibit favourable properties even in this more complicated setting of the heteroskedastic transformation model. However, our results are just indicative of the performance of existing tests in this setting, and a more in depth study is needed to explore the theoretical properties of these tests, which might shed more light on some of the prevailing issues mentioned above.

urn

6. Illustrative applications

Jo

For our first application of the described procedures we consider the ultrasonic calibration data given in NIST/SEMATECH e-Handbook of Statistical Methods (the data can be downloaded from http://www.itl.nist.gov/div898/handbook/pmd/ section6/pmd631.htm). The response variable Y represents ultrasonic response and the predictor variable X is metal distance. We investigate the appropriateness of four alternative models: a homoskedastic model with or without transformation of the response variable and a heteroskedastic model with or without transformation of the response variable. For each of these models we tested for validity, i.e. independence of the error term and the regressor. 18

Journal Pre-proof

(G)

Table 7. Size and power results for assessing normality (∆n,w ) of the error terms appearing in Model B. The nominal size of the test is α = 0.05. w(t) = (1 + t2 /c2 )−1

ν

n

c = 0.1

c = 0.25 c = 0.5 c = 1

c = 0.5 c = 1

w(t) = exp(−c|t|)

c = 1.5 c = 2

3.5 4.0 4.2

3.4 4.1 4.3

3.7 4.4 4.6

3.7 4.7 4.8

2.5 3.8 3.3

2.4 3.5 3.2

2.6 3.7 3.2

2.5 3.7 3.3

10

100 200 300

12.6 25.0 32.8

14.0 26.1 33.2

14.5 25.9 33.0

14.2 25.6 32.6

12.3 23.0 26.6

12.8 24.3 29.5

12.9 24.9 30.8

13.0 25.5 31.6

5

100 200 300

17.7 38.6 54.8

19.3 38.4 55.2

19.6 38.9 54.1

19.5 37.6 52.8

17.6 31.2 42.2

18.4 33.5 46.3

18.8 34.6 48.6

3

100 200 300

25.4 51.9 64.0

26.4 50.6 62.7

25.9 50.4 62.2

25.0 50.5 61.1

19.9 38.7 51.6

21.9 42.6 54.9

22.8 44.4 57.0

2

100 200 300

34.7 55.6 68.3

33.5 52.5 65.9

31.8 51.3 63.6

30.3 50.1 62.2

2.6 3.9 3.6

2.4 3.9 3.4

2.5 3.7 3.5

2.5 3.7 3.3

12.4 25.6 31.9

12.5 25.8 32.4

13.1 26.0 32.6

12.9 25.1 31.1

19.4 34.9 49.9

19.8 35.6 52.2

20.4 35.8 52.4

20.5 35.7 51.1

19.0 35.2 48.8

23.7 45.7 58.7

23.6 49.7 62.8

24.9 49.5 62.6

24.6 48.4 60.9

24.0 45.8 57.8

35.3 56.0 74.5

35.4 55.3 72.9

33.9 52.9 68.7

31.1 49.1 64.0

Pr e25.4 41.8 56.0

28.4 45.1 60.5

c = 0.05 c = 0.1 c = 0.25 c = 0.5

p ro

∞ 100 200 300

of

w(t) = exp(−ct2 )

30.0 47.7 62.9

31.4 49.8 64.9

(G)

Table 8. Size and power results for assessing normality (∆n,w ) of the error terms appearing in Model D. The nominal size of the test is α = 0.05.

η

ν

n

c = 0.1

0

∞

100 200 300

3.2 2.9 4.2

3.2 2.8 3.9

0

5

100 200 300

5.8 5.8 6.9

2.1

100 200 300

w(t) = (1 + t2 /c2 )−1

c = 0.5 c = 1

w(t) = exp(−c|t|)

c = 1.5 c = 2

c = 0.05 c = 0.1 c = 0.25 c = 0.5

3.0 2.8 3.8

3.1 3.0 3.8

1.7 1.8 2.2

1.9 2.0 2.3

1.9 2.1 2.4

2.0 2.1 2.6

2.1 2.1 2.4

1.7 1.9 2.4

1.9 1.9 2.3

2.6 2.3 2.2

6.6 6.3 7.7

6.5 6.5 8.6

6.7 7.0 9.2

5.4 6.3 8.8

5.4 6.8 8.6

5.3 6.7 8.9

5.5 6.7 9.1

5.4 6.8 9.2

5.4 6.8 8.4

5.9 6.4 9.0

6.0 6.7 9.7

16.3 26.6 33.9

16.2 25.8 30.5

15.6 25.7 27.6

14.7 24.1 25.7

14.0 21.8 22.2

14.9 24.5 26.0

15.7 25.7 28.0

16.0 26.9 29.5

15.5 26.3 29.4

15.4 24.1 25.6

13.0 21.4 22.7

10.5 18.2 19.5

39.3 64.3 77.9

37.4 59.9 72.3

35.7 56.1 67.4

33.2 52.4 62.1

23.3 40.0 51.6

28.0 47.2 59.9

29.9 51.2 63.9

31.0 53.8 66.9

31.2 52.7 65.5

25.7 44.6 56.3

21.4 36.6 48.9

17.7 30.8 40.6

urn

100

2.1 100 200 300

c = 0.25 c = 0.5 c = 1

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0

al

w(t) = exp(−ct2 )

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(S)

Table 9. Size and power results for assessing symmetry (∆n,w ) of the error terms appearing in Model A. The nominal size of the test is α = 0.05. w(t) = (1 + t2 /c2 )−1

w(t) = exp(−ct2 ) η

ν

n

0

∞

100 200 300

6.0 4.4 5.0

5.8 4.9 5.4

6.5 5.3 5.2

6.4 5.2 5.1

6.0 6.0 6.9

5.9 5.9 6.8

5.7 5.9 6.7

5

∞

100 200 300

23.0 38.2 48.8

23.5 39.5 50.1

23.5 39.1 49.1

22.4 37.7 47.2

22.0 35.5 46.6

21.2 34.9 45.9

20.8 34.1 44.8

20

∞

100 200 300

27.4 47.7 59.9

29.5 47.9 59.7

28.7 46.4 58.0

27.3 44.9 56.5

26.3 43.4 59.3

25.9 43.1 59.0

0

5

100 200 300

8.0 7.6 6.7

7.4 8.1 7.3

9.1 8.6 8.0

9.1 8.6 8.1

8.0 8.1 7.3

2.1 100 200 300

9.3 11.6 14.1

9.8 12.8 15.1

10.2 12.1 14.6

9.9 11.6 13.5

2.1

39.0 52.7 53.2

40.2 49.6 49.6

37.9 47.7 47.3

34.0 46.5 45.5

100

100 200 300

c = 1.5 c = 0.5 c = 1

w(t) = exp(−c|t|)

c = 1.5 c = 2

c = 0.5 c = 1

c = 1.5 c = 2

5.9 5.9 6.8

6.3 5.0 5.0

6.6 4.6 5.3

6.7 4.8 5.2

20.1 32.8 44.0

20.8 33.9 45.5

22.6 37.1 48.8

23.4 39.6 49.5

24.0 39.2 49.5

24.8 42.6 58.3

24.0 42.6 57.7

24.8 43.4 58.1

27.2 45.9 60.9

28.5 47.6 59.6

28.8 47.2 58.8

7.9 7.9 6.9

7.6 7.8 7.0

7.4 7.7 6.9

7.7 7.7 7.0

8.2 8.1 6.5

8.5 8.1 6.9

8.4 8.0 7.3

8.6 10.6 12.2

8.0 10.3 11.9

8.0 10.3 11.8

7.6 9.9 11.5

7.9 10.3 11.7

9.1 12.0 13.4

9.7 12.3 14.2

10.0 12.5 14.6

38.4 51.8 54.5

38.4 53.3 56.1

38.6 54.3 58.1

38.7 55.2 59.6

38.9 55.0 58.9

38.2 52.2 54.2

38.7 50.9 51.2

38.5 49.1 48.9

p ro

of

5.4 5.9 6.7

Pr e-

0

c = 0.25 c = 0.5 c = 1

(S)

Table 10. Size and power results for assessing symmetry (∆n,w ) of the error terms appearing in Model B. The nominal size of the test is α = 0.05. w(t) = exp(−ct2 ) n

c = 0.25 c = 0.5 c = 1 6.2 5.3 5.2

6.0 5.9 5.9

10

100 200 300

16.8 28.7 42.2

19.1 29.9 43.3

5

100 200 300

26.9 45.6 56.6

3

100 200 300

2

100 200 300

c = 1.5 c = 2

w(t) = exp(−c|t|) c = 0.5 c = 1

c = 1.5 c = 2

6.5 6.0 5.9

6.0 5.9 5.9

5.3 6.3 6.7

5.4 6.3 6.8

5.4 6.1 6.9

5.4 6.0 6.8

5.5 6.2 6.9

6.1 6.5 5.2

6.2 5.6 5.7

6.4 5.7 5.9

19.1 30.4 43.4

18.0 29.9 42.8

15.6 26.4 38.0

15.3 26.5 37.5

15.0 25.9 36.2

14.5 25.2 35.5

14.8 25.9 36.6

16.2 29.2 41.6

18.0 28.9 42.6

18.4 30.2 43.3

28.5 46.0 56.0

28.7 45.3 54.9

26.5 44.2 53.3

26.0 43.3 55.1

25.1 42.7 55.1

24.1 41.9 54.3

23.2 41.4 53.6

24.4 42.4 54.3

26.2 44.2 56.7

27.4 45.6 56.0

27.8 45.6 55.4

36.5 55.9 66.1

37.5 55.8 64.6

35.9 54.5 62.4

33.0 53.0 60.1

34.9 55.4 63.9

34.8 55.5 64.1

34.2 55.8 63.7

34.0 55.5 63.5

35.0 55.9 63.8

35.8 55.8 65.7

36.3 55.7 64.9

35.8 55.2 63.5

41.0 60.3 66.9

41.1 58.6 64.6

39.9 55.9 61.1

35.7 53.4 59.4

38.5 58.4 66.7

38.3 59.2 67.0

37.8 59.3 67.5

37.5 59.4 68.4

37.5 59.8 68.3

39.1 60.1 67.2

40.9 59.3 65.1

40.4 57.9 63.8

Jo

urn

∞ 100 200 300

c = 1.5 c = 0.5 c = 1

al

ν

w(t) = (1 + t2 /c2 )−1

20

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(S)

w(t) = (1 + t2 /c2 )−1

n

1

100 200 300

c = 0.25 c = 0.5 c = 1 7.2 7.2 5.6

7.6 7.6 6.9

8.7 7.7 7.6

c = 1.5 c = 0.5 c = 1 8.8 8.8 7.9

8.8 8.3 7.6

8.8 8.0 7.7

2

100 200 300

25.8 37.1 49.8

25.8 35.7 46.7

24.7 34.8 45.5

22.8 32.9 42.1

23.0 33.7 48.8

23.2 33.9 49.7

5

100 200 300

29.8 45.8 54.2

30.3 44.6 53.3

29.6 42.9 51.2

27.1 41.9 48.6

27.2 43.6 48.2

27.4 43.5 47.9

c = 1.5 c = 2

c = 0.5

c=1

c = 1.5

c=2

8.6 8.2 7.5

9.2 7.3 5.4

7.5 7.5 6.4

8.1 7.5 7.7

21.8 34.0 49.3

22.7 35.1 51.3

24.4 35.5 50.3

25.5 36.0 48.1

25.4 35.6 47.0

26.9 43.9 50.4

27.4 44.1 50.4

29.2 45.1 54.6

29.5 45.0 53.7

29.5 44.2 53.5

8.5 7.9 7.7

8.4 7.7 7.4

22.4 33.8 49.0 27.1 43.8 49.5

Pr e-

κ

w(t) = exp(−c|t|)

p ro

w(t) = exp(−ct2 )

of

Table 11. Size and power results for assessing symmetry (∆n,w ) of the error terms appearing in Model C. The nominal size of the test is α = 0.05.

(S)

Table 12. Size and power results for assessing symmetry (∆n,w ) of the error terms appearing in Model D. The nominal size of the test is α = 0.05. w(t) = exp(−ct2 ) ν

n

0

∞

100 200 300

3.6 4.4 5.0

3.8 4.4 5.0

0

5

100 200 300

5.9 5.9 5.8

6.4 6.1 6.0

8.2 8.0 8.8

40.3 53.4 56.6

2.1

100 200 300

w(t) = exp(−c|t|)

c = 1.5 c = 2

c = 0.5 c = 1

c = 1.5 c = 2

4.1 4.0 4.8

3.9 4.1 4.8

3.6 4.2 4.8

3.6 4.1 4.8

3.3 4.0 4.8

3.4 4.1 4.8

3.7 4.4 5.0

3.8 4.4 5.0

3.9 4.1 4.8

6.9 5.9 5.9

6.5 5.7 5.6

5.8 5.7 5.3

5.4 5.6 5.3

5.2 5.6 5.2

5.1 5.5 5.0

5.1 5.6 5.2

5.8 5.8 5.6

6.4 6.0 5.7

6.7 6.1 5.9

8.1 8.0 9.0

7.7 7.3 8.6

7.7 7.1 7.8

7.8 7.3 7.7

7.7 7.2 8.0

7.6 7.1 7.8

7.5 6.7 7.7

7.7 7.0 8.0

8.0 7.9 8.1

8.1 7.9 8.4

7.9 7.6 8.5

39.0 50.4 52.7

36.9 47.2 49.0

35.0 45.4 46.7

39.6 54.7 58.6

40.1 56.9 61.2

40.1 59.1 63.7

41.3 60.1 65.3

41.0 59.8 64.9

39.8 53.8 57.4

39.1 51.0 53.8

38.7 49.7 51.4

urn

100

2.1 100 200 300

c = 1.5 c = 0.5 c = 1

4.1 4.4 4.8

Jo

0

c = 0.25 c = 0.5 c = 1

al

η

w(t) = (1 + t2 /c2 )−1

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Pr e-

p ro

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We employed all tests considered in this paper (and their homoskedastic counterparts introduced by Neumeyer et al., 2016, and Hušková et al., 2018), and for all tests based on the characteristic function we chose a Gaussian characteristic kernel. The choice of the tuning parameter was based on the Monte Carlo study and is shown in Table 13 along with the numerical results. For this application we used 1 000 bootstrap replications and assume a significance level of 0.05. For simplicity we used the rule of thumb of Fan and Gijbels (1996, p. 111) (implemented in the package locpol by Cabrera, 2018) for regression bandwidths coupled with Silverman (1986, equations (3.28) and (3.30) on pp. 45 and 47) for density estimation bandwidths. For the homoskedastic case the results indicate a poor fit of the respective nontransformation models, but implementation of the Box–Cox transformation on the response clearly improves the fit according to all tests. An enhanced fit for the after-transformation model is also illustrated by the results corresponding to the heteroskedastic case although in this case the model can not be rejected even before transformation. As our second application we consider the heteroskedastic location-scale model for the Canadian cross-section wage data and the Italian GDP data; these data are also discussed in Racine and Li (2017) in the context of the non-transformation model. For the Canadian wage data there are n = 205 observations with ‘age’ considered as predictor for ‘logwage’. For the Italian GDP data there are n = 1008 observations with ‘year’ considered as predictor for ‘GDP’. Our findings (see Table 14) show that for the Canadian wage data the introduction of the transformation model seems again to improve the fit according to the KS and CM tests, while the CF-based test is robust in this respect and indicates a non-fit. For the Italian data however quite the opposite holds: The KS and CM tests indicate that neither the non-transformation nor the transformation model is appropriate, while the CF-based test shows a remarkably improved fit that clearly favours the transformation model. These results are partly in line with Racine and Li (2017) as they also find an insignificant KS statistic for the Canadian wage data but at the same time reject the location-scale presumption for the Italian data. On the other hand our findings indicate that while performing a Box–Cox transformation on the response might still lead to the same conclusion, there exist cases where this transformation could enhance the fit of the underlying model. 7. Conclusions

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New tests for the validity of the heteroskedastic transformation model are proposed which are based on the well known factorisation property of the joint characteristic function into its corresponding marginals. The asymptotic null distribution 22

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Table 13. Estimates of the transformation parameter under the four considered models when applied to the ultrasonic calibration data, along with the p-values of the tests for model validity. Homoskedastic case No transformation

Box–Cox

n/a

b = 0.458 ϑ

Parameter estimate Test for validity

KS CM ∆n,W (c = 1)

0.055 0.002 0.025

No transformation

Box–Cox

n/a

b = −0.436 ϑ

of

Test

Heteroskedastic case

0.366 0.138 0.403

0.347 0.363 0.132

0.751 0.568 0.294

p ro

Table 14. Results for the Canadian cross-section wage data and Italian GDP data, along with the p-values of the tests for model validity. Canadian wage data Test

No transformation

Test for validity

n/a KS CM ∆n,W (c = 1)

0.198 0.074 0.021

No transformation

Box–Cox

b = 2.842 ϑ

n/a

b = −0.202 ϑ

0.331 0.128 0.020

Pr e-

Parameter estimate

Italian GDP data

Box–Cox

<0.001 <0.001 <0.001

<0.001 <0.001 0.960

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Acknowledgements

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is derived and the consistency of the new criteria is shown. A Monte Carlo study is included by means of which a resampling version of the proposed method is compared to earlier methods and shows that the new test, aside from being computationally convenient, compares well and often outperforms its competitors, particularly under heavy tailed error distributions. A further Monte Carlo study of characteristicfunction based tests for symmetry and normality of regression errors exhibit analogous favourable features. Finally a couple of illustrative applications on real data lead to interesting conclusions.

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The authors wish to sincerely thank the Editor and an Associate Editor for timely handling the manuscript, and two anonymous referees for their constructive criticism that led to improvement of the paper. The work of the first author was partially supported by the grant GAČR 18-08888S. The work of the third author was partially supported by OP RDE project No. CZ.02.2.69/0.0/0.0/16_027/0008495, International Mobility of Researchers at Charles University.

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8. Appendix

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8.1. Assumptions We start with formulation of the assumptions and then we give the assertions on behavior of the test statistics under both the null hypothesis and some alternatives.

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(A.1) (Yj , Xj ), j = 1, . . . , n, are i.i.d. random vectors, where the covariates Xj , j = 1, . . . , n, have a compact support RX with RX ⊂ Rp . Q (A.2) We use a product kernel K(y) = ps=1 k(ys ), y = (y1 , . . . , yp )> , with k(·) being symmetric and continuous in [−1, 1], and satisfying Z 1 Z 1 r up+1 k(u)du 6= 0, u k(u)du = δr,0 , r = 0, . . . , p, −1

−1

where δr,s stands for Kronecker’s delta.

(A.3) The bandwidth h = hn satisfies

(nhp )−1 + nhp+δ → 0, for some δ > 0.

as n → ∞

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(A.4) It holds that E||Xj ||2 < ∞, and that Xj has a density f (·) satisfying

urn

0 < inf f (x) ≤ sup f (x) < ∞, x∈RX x∈RX f (x) − L(f, y, x − y, s1 ) ≤ ||x − y||s1 +δ1 d1 (y),

where L(f, y, x − y, s1 ) is the Taylor expansion of the density f of order s1 at x, δ1 > 0 and E|d1 (Xj )|2 < ∞, for some s1 + 1 ≥ p/2.

Jo

(A.5) It is assumed mϑ0 (x), σϑ0 (x), x ∈ RX satisfy mϑ0 (x) − Lϑ0 (m, y0 , x − y0 , s2 ) ≤ ||x − y0 ||s2 +δ2 d2 (y0 ), σϑ0 (x) − Lϑ0 (σ, y0 , x − y0 , s3 ) ≤ ||x − y0 ||s3 +δ2 d3 (y0 ) 28

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of

where Lϑ0 (mϑ0 , y0 , x − y0 , s2 ) is the Taylor expansion of regression function mϑ0 of order s2 at y0 and E|d2 (Xj )|2 < ∞ for some s2 ≥ p/2 and Em2ϑ0 (Xj ) < ∞. Similarly, Lϑ0 (σϑ0 , y0 , x − y0 , s3 ) is the Taylor expansion of regression function σϑ0 of order s3 at y0 and E|d3 (Xj )|2 < ∞ for some s3 ≥ p/2 and Eσϑ2 0 (Xj ) < ∞. (A.6) L = {Tϑ ; ϑ ∈ Θ} is a parametric class of strictly increasing transformations, Θ is a open measurable subset of Rq , and for some ξ > 0,

p ro

q X ∂Tϑ (Yj ) (ϑ − ϑ ) T (Y ) − T (Y ) − ϑ j /||ϑ − ϑ0 ||1+δ4 s s0 ϑ0 j ∂ϑ ϑ=ϑ 0 s ||ϑ−ϑ0 ||≤ξ s=1

sup

≤ d4 (Yj ),

Pr e-

where Ed24 (Yj ) < ∞, E(|d4 (Yj )||Xj ) < ∞, a.s., and for some δ4 > 0 it holds that  ∂E T (Y )|X
 ∂T (Y ) ϑ j j ϑ j E , a.s., Xj = ∂ϑs ϑ=ϑ0 ∂ϑs ϑ=ϑ0 2   ∂T (Y ) ϑ j < ∞, ETϑ2 (Yj ) < ∞. E E Xj ∂ϑs ϑ=ϑ0

b of ϑ0 (they are q-dimensional) satisfies: (A.7) The estimator ϑ

al

n  √  1 X b g(Yj , Xj ) + oP (1) n ϑ − ϑ0 = √ n j=1

where g(Yj , Xj ) has zero mean and finite covariance matrix.

urn

(A.8) The weight function is such that W (t1 , t2 ) = w1 (t1 )w2 (t2 ), where wm (·) satisfy Z ∞ t2 w1 (t)dt < ∞, w2 (t) = w2 (−t), t ∈ Rp . w1 (t) = w1 (−t), t ∈ R, −∞

Jo

(A.9) L = {Tϑ ; ϑ ∈ Θ} is a parametric class of strictly increasing transformations, Θ is a open measurable subset of Rq and that for all ϑ ∈ Θ |Tϑ (Yj ) − Tϑ0 (Yj )| ≤ d5 (Yj )||ϑ − ϑ0 ||,

with E|d5 (Yj )| < ∞. 29

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Comments on the assumptions: • Assumptions (A.2) are (A.3) are quite standard. • Assumption (A.4) requires smoothness of the density f (·) of X.

p ro

of

• Assumption (A.5) formulates
the requirements on the regression function mϑ0 (x) = E Tϑ (Yj )|Xj = x . Motivation for assumptions (A.4) and (A.5) are from Delgado and González-Manteiga (2001). √ • Assumption (A.7) requires that a n-estimator of ϑ0 with an asymptotic representation is available. Such estimators are proposed and studied in, e.g., Breiman and Friedman (1985), Horowitz (2009), Linton et al. (2008) and Neumeyer et al. (2016). They are either based on a modified least squares method or on profile likelihood estimators or on the mean square distance from independence.

Pr e-

• Assumptions (A.9) and (A.10) are for the considered class of alternatives. 8.2. Proofs The proofs are quite technical and therefore we present the main steps only. Additionally the main lines of proof follow those in Hlávka et al. (2011). However some modifications and extensions are needed. Standard technique from nonparametric regression is applied together with functional central limit theorems. When there is no ambiguity we use the following simpler notations: εbj = εbϑ,j b ,

m b n (x) = m b ϑb(x),

al

εj = εϑ0 ,j ,

σ bn (x) = σ bϑb(x).

For simplicity we give the proofs only for ϑ univariate. Multivariate situation proceeds quite analogously.

Jo

urn

Proof of Theorem 1. By assumption (A.7) and elementary properties of the functions sine and cosine Z Z ∆n,W = |Jn,1 (t1 , t2 ) + Jn,2 (t1 , t2 )|2 W (t1 , t2 )dt1 dt2 , (14)

30

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where
1 Xh Jn,1 (t1 , t2 ) = √ cos(t1 εbj ) − E cos(t1 εj ) g+ (t> 2 Xj ) n j=1 i
> + sin(t1 εbj ) − E sin(t1 εj ) g− (t2 Xj ) , Jn,2 (t1 , t2 ) =

n n 1 XXh

n3/2

j=1 v=1

of

n


cos(t1 εbv ) − E cos(t1 εv ) g+ (t> 2 Xj )

with

p ro

i
+ sin(t1 εbv ) − E sin(t1 εv ) g− (t> X ) j 2

Pr e-


> > > > g+ (t> 2 X) = cos(t2 X) + sin(t2 X) − E cos(t2 X) + sin(t2 X) ,
> > > > g− (t> 2 X) = cos(t2 X) − sin(t2 X) − E cos(t2 X) − sin(t2 X) .

A useful asymptotic representation for Jn,1 (t1 , t2 ) is provided in Lemma 1 below, while negligibility of Jn,2 (t1 , t2 ) is proved quite analogously and its proof is therefore omitted. Lemma 1. Let the assumptions of Theorem 1 be satisfied. Then, as n → ∞, Z Z 2 P Jn,1 (t1 , t2 ) − Qε,X,c (t1 , t2 ) − Qε,X,s (t1 , t2 ) − Lε,X (t1 , t2 ) W (t1 , t2 )dt1 dt2 → 0,

al

where

n

Jo

urn

1 X (cos(t1 εj ) − E cos(t1 εj ))) + t1 εj Sε (t1 ) Qε,X,c (t1 , t2 ) = √ n j=1 + t1 (ε2j − 1)Cε0 (t1 )/2 g+ (t> 2 Xj ), n
1 X Qε,X,s (t1 , t2 ) = √ (sin(t1 εj ) − E sin(t1 εj ) ) − t1 εj Sε (t1 ) n j=1 − t1 (ε2j − 1)Cε0 (t1 )/2 g− (t> 2 Xj ), √ Lε,X (t1 , t2 ) = n(ϑb − ϑ0 )Hϑ ,1 (t1 , t2 ) 0

where Cε and Sε are the real and the imaginary part of the CF of εj and Cε0 and Sε0 are respective derivatives and Hϑ0 ,1 (t1 , t2 ) is defined in Theorem 1 with q = 1. 31

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of

Proof. Recall that the residuals εbj can be expressed as   mϑ0 (Xj ) − m b n (Xj ) Tϑ (Yj ) − Tϑ0 (Yj ) σϑ0 (Xj ) −1 + + , j = 1, . . . , n, εbj = εj + εj σ bn (Xj ) σ bn (Xj ) σ bn (Xj ) (15) and then by Taylor expansion and smoothness of Tϑ (Yj ) w.r.t. ϑ we have cos(t1 εbj ) = cos(t1 εj )

p ro

 h  σ (X ) mϑ0 (Xj ) − m b n (Xj ) Tϑb(Yj ) − Tϑ0 (Yj ) i ϑ0 j − t1 sin(t1 εj ) εj −1 + + σ bn (Xj ) σ bn (Xj ) σ bn (Xj ) 2 c + t1 Rnj (t1 ), (16)

Pr e-

c j = 1, . . . , n, where Rnj (t)’s are remainders. Similar relations can be obtained for sin(tb εj ). Comparing the present situation with that considered in Hlávka et al. (2011) there is an additional parameter ϑ which influences the behavior of the treated variables. We notice

al

m b ϑ (Xj ) − mϑ0 (Xj ) n   X − X  X 1 v j = Tϑ (Yv ) − Tϑ0 (Yv ) K h nhp fb(Xj ) v=1 n   X − X  X 1 v j εv σϑ0 (Xv ) + mϑ0 (Xv ) − mϑ0 (Xj ) + K h nhp fb(Xj ) v=1

Jo

urn

:= A1,n (Xj , ϑ) + A2,n (Xj , ϑ), say,

32

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and σ bϑ2 (Xj )

X  Xv − Xj  1 K Tϑ (Yv ) − Tϑ0 (Yv ) + εv σϑ0 (Xj ) h nhp fb(Xj ) v=1 2 + mϑ0 (Xv ) − mϑ0 (Xj ) n

of

=

Pr e-

p ro

n  X − X  2 X 1 v j K Tϑ (Yv ) − Tϑ0 (Yv ) = h nhp fb(Xj ) v=1 n  X − X  2 X 1 v j K εv σϑ0 (Xj ) + mϑ0 (Xv ) − mϑ0 (Xj ) + h nhp fb(Xj ) v=1 n  X − X   X 1 v j K +2 Tϑ (Yv ) − Tϑ0 (Yv ) εv σϑ0 (Xv ) h nhp fb(Xj ) v=1 n  X − X    X 1 v j K Tϑ (Yv ) − Tϑ0 (Yv ) mϑ0 (Xv ) − mϑ0 (Xj ) +2 h nhp fb(Xj ) v=1

:= B1,n (Xj , ϑ) + B2,n (Xj , ϑ) + B3,n (Xj , ϑ) + B4,n (Xj , ϑ), say.

Z Z 

al

The terms A1,n (Xj , ϑ), A2n (Xj , ϑ), B2,n (Xj , ϑ), B3,n (Xj , ϑ) are influential while the others are properly negligible. The desired property for A2n (Xj , ϑ), B2,n (Xj , ϑ) follow from results in Hlávka et al. (2011) and are formulated below, the remaining terms will be discussed shortly later. Proceeding as in Hlávka et al. (2011) we find that

urn

n n
1 X √ g+ (t2 Xj ) (cos(t1 εj ) − E cos(t1 εj ) Qε,X,c (t1 , t2 ) − n j=1 h −A (X , ϑ) ε  B (X , ϑ) io2 P 2,n j j 2,n j − t1 sin(t1 εj ) − −1 W (t1 , t2 )dt1 dt2 → 0, 2 σϑ0 (Xj ) 2 σϑ0 (Xj )

Jo

Next we study the remaining influential terms, i.e., terms that depends on ϑ. We

33

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start with n  1 X −A1,n (Xj , ϑ) − εj B3,n (Xj , ϑ) + Tϑ (Yj ) − Tϑ0 (Yj )  √ g+ (t2 Xj ) − t1 sin(t1 εj ) σϑ0 (Xj ) n j=1

h 1 X 1 =√ g+ (t2 Xj )(−t1 sin(t1 εj )) Tϑ (Yj ) − Tϑ0 (Yj ) σϑ0 (Xj ) n j=1

of

n

X  Xv − Xj 

i 1 K Tϑ (Yv ) − Tϑ0 (Yv ) 1 + εv εj − h nhp fb(Xj ) v=1 n n  1 X X  Xv − Xj  K =√ Tϑ (Yj ) − Tϑ0 (Yj ) h n j=1 v=1 h 1 × (g+ (t2 Xj )(−t1 sin(t1 εj )) nhp fb(Xj )σϑ0 (Xj ) i 1 − g+ (t2 Xv ))(−t1 sin(t1 εv ))(1 + εj εv ) . nhp fb(Xv )σϑ (Xv )

Pr e-

p ro

n

0

This behaves asymptotically as (due to assumption (A.7)) √

n(ϑb − ϑ0 ) ×

h

n n 1 X X  Xv − Xj  ∂Tϑ (Yj ) K n j=1 v=1 h ∂ϑ ϑ=ϑ0

1

al

(g+ (t2 Xj )(−t1 sin(t1 εj )) nhp fb(Xj )σϑ0 (Xj ) i 1 − g+ (t2 Xv ))(−t1 sin(t1 εv ))(1 + εjθ0 εv ) nhp fb(Xv )σϑ (Xv ) 0

urn

By the same assumption it follows that, to study

√ b n(ϑ−ϑ0 ) = OP (1), and hence it suffices

n 1 X ∂Tϑ (Yj ) 1 C1,n (t1 , t2 ) = g+ (t2 Xj )(−t1 sin(t1 εj )) n j=1 ∂ϑ ϑ=ϑ0 σϑ0 (Xj )

n n 1 X X  Xv − Xj  ∂Tϑ (Yj ) 1 K g+ (t2 Xv ) n j=1 v=1 h ∂ϑ ϑ=ϑ0 nhp fb(Xv )σϑ0 (Xv )

Jo

C2,n (t1 , t2 ) = −

× (−t1 sin(t1 εv ))(1 + εj εv )

34

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By the law of large numbers (uniform in t1 , t2 ), as n → ∞,

of

 ∂T (Y )  1 P ϑ 1 C1,n (t1 , t2 ) → E g+ (t2 X1 )(−t1 sin(t1 ε1 )) , ∂ϑ ϑ=ϑ0 σϑ0 (X1 )    ∂T (Y ) 1 P ϑ 1 (1 + ε1 ε2 ) − t1 sin(t1 ε2 )g+ (t> X ) . C2,n (t1 , t2 ) → −E 2 2 ∂ϑ ϑ=ϑ0 σϑ0 (X2 )

Similarly we proceed with

p ro

n  1 X −A1,n (Xj , ϑ) − εj B3,n (Xj , ϑ) + Tϑ (Yj ) − Tϑ0 (Yj )  √ g− (t2 Xj ) t cos(tεj ) σϑ0 (Xj ) n j=1

and the proof of Lemma 1 is finished.

Jo

urn

al

Pr e-

Proof of Theorem 2. We proceed as in the proof of Theorem 1 and Lemma 1 and come to the conclusion that Z 1 P ∆n,W → |ϕX,εϑ0 (t2 , t1 ) − ϕX (t2 )ϕεϑ0 (t1 )|2 W (t1 , t2 )dt1 dt2 > 0. n p R

35