Journal Code: S J O S Article ID 1 2 0 7 9 Dispatch 30.01.14 No. of Pages: 18 CE: Patrimonio, Cheryl F. ME: Scandinavian Journal of Statistics doi: 10.1111/sjos.12079 © 2014 Board of the Foundation of the Scandinavian Journal of Statistics. Published by Wiley Publishing Ltd. ALBERT VEXLER PR OO F Computing Critical Values of Exact Tests by Incorporating Monte Carlo Simulations Combined with Statistical Tables Department of Biostatistics, The University at Buffalo, State University of New York YOUNG MIN KIM Department of Biostatistics, The University at Buffalo, State University of New York JIHNHEE YU Department of Biostatistics, The University at Buffalo, State University of New York NICOLE A. LAZAR Department of Statistics, University of Georgia ED ALAN D. HUTSON Department of Biostatistics, The University at Buffalo, State University of New York CO R RE CT ABSTRACT. Various exact tests for statistical inference are available for powerful and accurate decision rules provided that corresponding critical values are tabulated or evaluated via Monte Carlo methods. This article introduces a novel hybrid method for computing p-values of exact tests by combining Monte Carlo simulations and statistical tables generated a priori. To use the data from Monte Carlo generations and tabulated critical values jointly, we employ kernel density estimation within Bayesian-type procedures. The p-values are linked to the posterior means of quantiles. In this framework, we present relevant information from the Monte Carlo experiments via likelihood-type functions, whereas tabulated critical values are used to reﬂect prior distributions. The local maximum likelihood technique is employed to compute functional forms of prior distributions from statistical tables. Empirical likelihood functions are proposed to replace parametric likelihood functions within the structure of the posterior mean calculations to provide a Bayesian-type procedure with a distribution-free set of assumptions. We derive the asymptotic properties of the proposed nonparametric posterior means of quantiles process. Using the theoretical propositions, we calculate the minimum number of needed Monte Carlo resamples for desired level of accuracy on the basis of distances between actual data characteristics (e.g. sample sizes) and characteristics of data used to present corresponding critical values in a table. The proposed approach makes practical applications of exact tests simple and rapid. Implementations of the proposed technique are easily carried out via the recently developed STATA and R statistical packages. UN 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 Key words: Bayesian point estimation, empirical likelihood, exact tests, local maximum likelihood, posterior expectation, quantile estimation 1. Introduction Exact tests are well known to be simple, efﬁcient and reliable statistical tools in a variety of applications. The statistical literature has extensively addressed many parametric, semiparametric and nonparametric exact tests that have ﬁnite sample type I error control, for example, exact tests for quantiles (Serﬂing, 2002, p. 102), Fisher’s exact test, the exact F -test, the Shapiro–Wilk test of normality, the Wilcoxon rank-sum test, Hall and Welsh’s test for normality (Hall & Welsh, 1983) and the exact test for comparing multivariate distributions 2 Scand J Statist CO R RE CT ED PR OO F proposed by Rosenbaum (2005), to name just a few. Many of these tests are now found in standard statistical software packages, for example, SPSS. Exact tests are not only useful in the small sample size setting but also invaluable in the case of rare events in the large sample setting; for example, see Mudholkar & Hutson (1997) for the case of contingency tables and the inaccuracy of asymptotic methods when there is a large number of zero count cells. A major disadvantage is that, in general, exact tests are computationally intensive and may not be feasible for moderate to large sample sizes. In these instances, Monte Carlo (MC) methods may provide accurate and computationally feasible approximations. Commonly, tables of corresponding critical points are required for the exact tests’ implementation. Because all possible critical values cannot be tabulated, two different approaches can be employed in practice to calculate the exact p-value. One method is related to interpolation/extrapolation of the relevant critical points on the basis of tabulated values. The use of tables with corresponding critical values is a standard method applied in various statistical software routines. Employing a table-based methodology for use within the testing algorithm is quick and efﬁcient. However, the interpolation/ extrapolation method using table-based methods becomes much less reliable when real data characteristics (e.g. sample sizes) differ from those used to tabulate the critical values. Alternatively, as a second approach, one can conduct MC simulations to approximate the exact p-values. The approach based on MC simulations is simple and accurate but oftentimes computationally intensive. In this article, we propose and examine a novel Bayesian hybrid method to compute p-values of exact tests on the basis of both statistical tables of related critical values and MC simulations. The key feature of the proposed method is that if actual data characteristics (e.g. sample sizes) are close to those used in the table of critical values, then relatively few MC simulations are needed to improve the accuracy of p-value calculations. When this is not the case, more MC simulations are necessary. Consider the following simple illustration. Table 1 represents a part of ‘percentiles of the 2 distribution’ (Kutner et al., 2005) at 29, 30, 40 and 50 degrees of freedom. Suppose that we are interested in the percentiles of the 2 distribution with 35 degrees of freedom and level of signiﬁcance 0.05. Table 1 does not provide an exact percentile in this case. Interpolation between the 30-degree-of-freedom and 40-degree-of-freedom entries or an MC procedure based on randomly generated 2 values can accurately approximate the exact (missing and unknown) percentile. In another scenario, suppose we are interested in the case with 31 degrees of freedom and 0.05 level of signiﬁcance. In this case, we could conduct an MC study with fewer replications than required in the previous case (35 degrees of freedom) because the desired percentile is nearer to the tabulated value at 30 degrees of freedom. The proposed Bayesian hybrid method calculates the percentile of interest by incorporating the information from Table 1 as a prior distribution to improve the accuracy and reduce the computational cost of the simulations. In this article, we consider a Bayesian solution to the problem through the incorporation of information from MC simulations and tabulated critical values. Hence we consider the tables to UN 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 T125 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 A. Vexler et al. Table 1. Percentiles of the 2 distribution (Kutner et al., 2005) where d.f. is degrees of freedom Level of signiﬁcance d.f. 0.025 0.050 0.100 29 30 40 50 14.26 14.95 22.16 37.48 16.05 16.79 24.43 40.48 17.71 18.49 26.51 43.19 © 2014 Board of the Foundation of the Scandinavian Journal of Statistics. Computing critical values of exact tests Scand J Statist CO R RE CT ED PR OO F represent prior information, whereas the likelihood part of the analysis is based on the output of the relevant MC simulations. Therefore, the two key issues that need to be addressed for the proposed hybrid method are as follows: (i) constructing the functional forms of the prior distribution based on of tabulated critical values; and (ii) developing nonparametric likelihoodtype functions from information extracted from MC simulations. In this work, we apply the local maximum likelihood (LML) technique (Tibshirani & Hastie, 1987; Fan & Gijbels, 1995, 1996; Fan et al., 1995; Fan et al., 1998) to obtain functional forms of prior distributions from tables of critical values. The LML methodology is usually applied to construct functional forms of dependency between variables. For example, in regression, the aim of the LML approach is to explore the association between dependent and independent variables when the regression functions are data driven instead of being limited to a certain presumed form (Fan & Gijbels, 1995). The ﬂexibility and relative simplicity of the LML methodology makes it a practical solution to the ﬁrst of our problems. As for the second problem, that is, construction of a likelihood to summarize the data based on MC generations of exact-test-statistic values, we propose the nonparametric technique of empirical likelihood (EL; e.g. Owen, 1988, 1990, 2001; Lazar & Mykland, 1998; Yu et al., 2010; Vexler & Gurevich, 2010; Vexler & Yu, 2011). It is well known that EL is a powerful nonparametric technique of statistical inference. Lazar (2003) further demonstrated that the EL functions can be used to construct distribution-free Bayesian posterior probabilities. Following the results of Lazar (2003), we develop the nonparametric posterior expectations of quantiles via EL in our proposed procedure for computing the p-values of exact tests. Evaluation of the exact p-value in our context requires statistical inference for quantiles. We achieve this inference using the EL technique based on kernel densities. As shown by Chen & Hall (1993), the use of kernel densities to smooth EL signiﬁcantly improves the performance of the EL ratio tests for quantiles, in terms of relative accuracy. Zhou & Jing (2003) proposed an alternative smoothed EL approach where the likelihood ratio has an explicit form based on Mestimators. Yu et al. (2011) also showed that the use of kernel densities signiﬁcantly improves two-sample EL ratio tests for medians. In order to implement our approach, we need to calculate the posterior expectation given the a priori information (tables of critical values) and the observed data (MC simulations). In the case of the classical Bayesian analysis, there are some instances where the integrals related to the posterior expectations may be evaluated analytically. More often than not, however, these calculations are intractable, and numerical methods are used to obtain a solution. Alternatively, this article provides an asymptotic expression for the proposed nonparametric posterior expectations, which utilizes the asymptotic Gaussian form of the posterior distribution of quantiles based on maximum likelihood estimation. Note that the proposed nonparametric posterior expectations are based on integrated EL functions. The EL functions themselves do not have closed analytical forms. Hence, we are required to use numerical methods. That is, computation of the posterior expectation in this instance is not trivial. In a similar manner to classical Bayesian inference (e.g. DasGupta, 2008; DiCiccio et al., 1997; Kass & Raftery, 1995), the asymptotic results developed in this article have good utility relative to the posterior expectation calculation, thus making our methodology tractable. The asymptotic approach also provides a method for calculating the required number of samples needed to generate speciﬁc MC simulations in order to obtain accurate critical values for the proposed exact tests. The criterion for computing the minimum number of MC repetitions is based on the prior information (tables of pre-populated critical values). This article is organized as follows. In Section 2, we describe the problem more fully and deﬁne our formal notation. We then deﬁne the nonparametric posterior expectation of the UN 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 3 © 2014 Board of the Foundation of the Scandinavian Journal of Statistics. 4 Scand J Statist CT ED PR OO F critical values based on EL with kernel functions. The asymptotic properties of the posterior expectation are shown, and the theoretical properties of the estimated critical values are evaluated asymptotically. Section 2 also presents the method to create functional forms of the prior distributions using the LML method based on statistical tables. Finally, we provide the procedure to calculate critical values of exact tests using our new method. Concluding remarks are presented in Section 3. Section S1 of the Supporting information demonstrates and examines the applicability of the proposed approach in practice. Note also that Tanajian et al. (2013) and Shepherd et al. (2013) have employed the proposed method in the STATA and R software packages, respectively, to apply new statistical testing strategies in practice. The STATA package provides nonparametric tests for symmetry of data distributions as well as comparing K-sample distributions. Recognizing the fact that recent statistical software packages do not sufﬁciently address K-sample nonparametric comparisons of data distributions, Tanajian et al. (2013) proposed a new STATA command VX_DBEL to execute exact tests on the basis of K-samples. To calculate p-values of the exact tests, the STATA command uses the following methods: (i) a classical technique based on MC p-value evaluations; (ii) an interpolation technique based on tabulated critical values; and (iii) a new hybrid technique that combines (i) and (ii). The second and third cutting edge methods are based on the methodology presented in this paper. The R package provides a function dbELnorm to be used for joint assessment of normality of K-independent samples with varying means and standard deviations. The function provides the test statistic and associated p-values. The p-values can be calculated by incorporating MC simulations combined with statistical tables in a manner similar to the technique considered in this paper. 2. Statements and methods CO R RE In this section, we formally state the problem related to the procedure development in order to compute critical values of exact tests using MC simulations and statistical tables. We begin with the following example. Suppose, for simplicity, that we have a two sample exact test 1˛ , where n; m are the respective sample sizes and ˛ is the level of with critical values qn;m 1˛ signiﬁcance. Let the values of qn;m be tabulated for n 2 N , m 2 M and ˛ 2 A, where N and M are sets of integer numbers and A is a set of real numbers from 0 to 1. That is, we have ® 1˛ ¯ the table deﬁned as qn;m I n 2 N; m 2 M; ˛ 2 A . We are interested in obtaining a value of ® ¯ 0 0 1˛ qn1˛ … qn;m I n 2 N; m 2 M; ˛ 2 A . There are two ways to evaluate qn1˛ : (i) a technique 0 ;m0 0 ;m0 ® 1˛ ¯ based on interpolation/extrapolation using the table values qn;m I n 2 N; m 2 M; ˛ 2 A ; and (ii) an approach based on a MC study for generating values of the test statistic. In this paper, ® 1˛ ¯ 0 we use the table qn;m I n 2 N; m 2 M; ˛ 2 A to construct a prior distribution for qn1˛ that 0 ;m0 will be combined through Bayes rule with a nonparametric likelihood function based on MC 0 generated values of the test statistic. The value of qn1˛ , the .1 ˛0 / 100% quantile, is then 0 ;m0 estimated from the posterior distribution that results. The next section presents the nonpara0 metric Bayesian approach to estimate qn1˛ using the posterior expectation of quantiles based 0 ;m0 on the smoothed EL method. Section 2.2 introduces the LML technique to derive a prior distribution of quantiles based on related statistical tables. In Section 2.3, the ﬁnal procedure of the hybrid method is provided to be applied to calculate critical values of exact tests efﬁciently. UN 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 A. Vexler et al. 2.1. Bayesian empirical likelihood evaluation of quantiles Smoothed empirical likelihood for quantiles. In this section, we denote the distribution-free posterior expectation of quantiles based on independent identically distributed observations X1 ; : : : ; Xt . Let Xj .j D 1; : : : ; t / denote a realization of the test statistic value at the j -th MC iteration when the MC simulations provide in total t generated values of the test statistic. The © 2014 Board of the Foundation of the Scandinavian Journal of Statistics. Computing critical values of exact tests Scand J Statist PR OO F distribution function, F; of X is unknown. We focus on the evaluation of the .1 ˛/ 100% quantile, Pr X < q01˛ D 1 ˛, ˛ 2 .0; 1/. To provide nonparametric statistical inference regarding q01˛ , we apply the nonparametric likelihood approach proposed by Chen & Hall (1993). Towards this end, denote K./ to be a smoothed version of the indicator function F0 deﬁned by F0 .u/ D 1, for u 0, 0 otherwise. Let k./ be a -th order kernel density function, commonly used in nonparametric density estimation (e.g. Silverman, 1986; Härdle, 1990). Then, the function K./ can be represented in the form Z k.u/du; (1) K.x/ D u<x where k./ satisﬁes Z 8 ˆ < 1 if j D 0; j u k.u/du D 0 if 1 j 1; ˆ : if j D ; CT ED for any integer 2 and a constant ¤ 0. We use the notation Kh ./ D K.= h/ and kh ./ D h1 k.= h/ as the kernel distribution and kernel density representations, respectively, with bandwidth h satisfying the following conditions: h ! 0 and th ! 1 as t ! 1. Write the smoothed EL function for quantiles as ˇ 2 3 ˇ t t t Y X X ˇ ® ¯ EL.q/ D sup 4 pj ˇˇ pj 0; pj D 1 and pj Kh q Xj .1 ˛/ D 05: ˇ j D1 j D1 j D1 (2) RE The method of Lagrange multipliers then can be used to ﬁnd the values of p1 ; : : : ; pt in (2) yielding the result: ¯1 ® pj D t C Kh q Xj .1 ˛/ ; (3) where , the Lagrange multiplier, is a root of Kh .q Xj / D 1 ˛: t C ¹Kh .q Xj / .1 ˛/º CO R t X j D1 (4) Chen & Hall (1993) showed that this smoothed EL for quantiles improves the coverage accuracy of conﬁdence intervals from order t 1=2 to order t 1 . Bayesian approach based on the smoothed empirical likelihood for quantiles. We assume that, in addition to the sample, X1 ; : : : ; Xt , we have prior information regarding the quantile q01˛ in the form of a density function denoted as .q/. Without loss of generality and for simplicity of notation, we also assume that the form of the density function of X1 is known to be f .xjq/. Classical Bayesian methodology yields the posterior probability of the quantile q01˛ as Qt j D1 f Xj jq .q/ PP .q/ D R 1 Qt : (5) j D1 f .Xj jq/ .q/ dq 1 UN 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 5 Thus, the posterior expectation of a given quantile is R1 Z1 E.qjX1 ; : : : ; Xt / D qPp .q/ dq D 1 Qt j D1 f .Xj jq/ .q/dq 1 q : R 1 Qt j D1 f .Xj jq/ .q/dq 1 © 2014 Board of the Foundation of the Scandinavian Journal of Statistics. (6) 6 Scand J Statist In this work, we replace in (5) with a nonparametric counterpart. Lazar (2003) proposed and validated the Bayesian EL method as an alternative to the classical parametric setting. This suggests the nonparametric form of (5) EL.q/ .q/ PNP .q/ D R X .t/ X.1/ EL .q/ .q/ dq (7) F ; R X.t/ X Z .t/ 1˛ qO NP D X qPNP .q/ dq D R X.1/ .t/ X.1/ X.1/ qEL.q/ .q/ dq : EL.q/ .q/ dq PR OO where X.1/ X.2/ : : : X.t / are the order statistics based on X1 ; : : : ; Xt and the function EL.q/ is deﬁned at (2), the kernel density-smoothed EL. It follows that the nonparametric posterior expectation of the (1-˛/th quantile is given by (8) RE CT ED Note that if, for example, for some q, we have maxj D1;:::;t ¹Kh .q Xj / .1 ˛/º < 0, then P there are no p1 ; : : : ; pt summing to 1 for which tj D1 pj ¹Kh .q Xj / .1 ˛/º D 0. If this happens, we deﬁne EL.q/ D 0 (Owen, 2001), and hence, by convention, and without loss of generality, we propose the support of the integrals in (7) and (8) to be restricted to ŒX.1/ ; X.t / . To investigate the asymptotic properties of the nonparametric posterior expectation of quantiles at (8), we assume that the data distribution function, F ./, is three times differentiable in a neighbourhood of q01˛ and that the applied kernel function is twice differentiable in a neighbourhood of q01˛ . Let us also deﬁne ´.r/ .q/ D d r ´.q/=dq r for P a smooth function ´./ and r D 3; 4; : : :, and kNh .q/ D t 1 tj D1 kh .q Xj /, kNh0 .q/ D P P .t h2 /1 tj D1 k 0 .h1 .q Xj // and KN s;h .q/ D t 1 tj D1 ¹Kh .q Xj / .1 ˛/ºs for s D 1; 2; 3. Then we have the following: Proposition 1. Let X1 ; : : : ; Xt be a random sample from a density function f ./. Suppose that q 2 1 2 the prior density is deﬁned as .q/ D exp .q 0 / .20 / 202 . Then, the asymptotic 1˛ qO NP D CO R approximation to the nonparametric posterior expectation of the (1-˛/-th quantile is given as 1˛ qO M C 1C 0 1˛ 02 Bt2 .q OM /t 1 1˛ 02 Bt2 .q OM /t 1˛ where qO M satisﬁes 1 ˛ D t 1 1˛ KN 2;h qO . UN 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 A. Vexler et al. C op 1 as t ! 1; t 1˛ 1˛ ® 1˛ ¯2 . 2 q O and B q O D kNh qO M K X h j t j D1 M M Pt M By proposition 1, one can show that the asymptotic distribution of the estimator (8) is p t Vt1=2 1˛ qO NP q01˛ ! p 1=2 t Vt 0 q01˛ d 1 1˛ 1C 2 2 1˛ ! N.0; 1/ as t ! 1; 0 Bt qO M 02 Bt2 qO M t t 1˛ where Vt D tVar qO M . The next proposition provides a method to control the accuracy of the proposed pvalues’ evaluations. To formulate the following result, we assume that F .x/ is an absolutely continuous cumulative distribution function with corresponding density function f .x/ and © 2014 Board of the Foundation of the Scandinavian Journal of Statistics. Computing critical values of exact tests Scand J Statist F 00 .x/ D f 0 .x/. We also assume that F .5/ q01˛ D f .4/ q01˛ exists. The application of the Taylor theorem then implies (9) F 1 2 1˛ 1˛ 1˛ D ˛ qO NP q01˛ f q01˛ q01˛ f 0 q01˛ qO NP 1 F qO NP 2 3 1 1˛ 1˛ 00 1˛ qO q0 f q0 C :::: 6 NP PR OO 1˛ 2 1˛ Q q01˛ f q01˛ C 0:5 qO NP q01˛ In this case, controlling values of .t/ D qO NP Q f 0 q01˛ , we can monitor the accuracy of the p-value evaluations. Here, ˇ ˇ .t/ may be thought ˇ ˇ Q of as a risk function. For example, the choice of t such that E .t / < (e.g. D 0:001/ provides the presumed accuracy of the q01˛ -estimation. This leads to the following: Proposition 2. The asymptotic representation of the expectation of the estimated probability at (9) is ED ° ± 1˛ Q / C o.t 1 /; E 1 F qO NP D ˛ E .t where CO R RE CT ´ d2 f 0 q01˛ h2 f q01˛ d4 f .3/ q01˛ h4 Q 1˛ 1˛ E .t/ D 2f q0 24f q0 1 C 2 f˛.1˛/ 2 q 1˛ t . 0 / 0 μ .1 C 2˛/ f q01˛ .d2 /2 f 0 q01˛ f 00 q01˛ h4 C C t 4f 2 q01˛ ´ 1˛ μ 1˛ 1˛ 0 f q0 ˛.1 ˛/f q0 ˛.1 ˛/ 0 q0 1˛ C 2f 3 q0 02 f 2 q01˛ t t 1 C 2 f˛.1˛/ 2 q 1˛ t . 0 / 0 ´ μ f 0 q01˛ ˛ .1 ˛/ ; 2 f 2 q01˛ t 2 1 C 2 f˛.1˛/ 2 q 1˛ t . 0 / 0 and d D R u k.u/du; D 2; 4: We outline the proofs of propositions 1 and 2 in the Appendix. Remark 1. Proposition 1 illustrates that the asymptotic result has a remainder term of 1˛ 1˛ order o.t 1 /. By virtue of proposition 1, we can easily show that qO NP D qO M C 2 2 1˛ 1 1˛ 1 0 qO M 0 Bt qO M t C op .t /. This indicates that we may asymptotically detect effects of the prior on the estimated quantile (8) with relative accuracy, because the term dependent on the prior information vanishes as t 1 . One can further show that if the kernel method is not used, the EL function could still be represented through the empirical distribution functions, for example, EL.q/ t.1 ˛ Ft .q//2 =.2˛.1 ˛//, where Ft .q/ D P t 1 tj D1 I.Xj q/ for an indicator function, I./. In this case, because of lack of smoothness of the estimator, the technique used to prove proposition 1 cannot be used because Ft is a step function. Instead, we can apply the classic Bahadur asymptotic results (e.g. Serﬂing, 2002, p. 93) ; these provide remainder terms calculated to be of order t 3=4 . This is not enough evidence to assess the asymptotic impact of the prior information in the evaluation of the distribution-free posterior expectation. UN 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 7 © 2014 Board of the Foundation of the Scandinavian Journal of Statistics. 8 Scand J Statist Remark 2. If X is discrete or has deﬁned discontinuities, we can reformulate the propositions below using the smoothing transformation given by Zj D Xj C "j , where "j N.0; 1/, j D 1; 2; : : : ; t, and ! 0 as t ! 1. PR OO F Remark 3. We suggest to choose a bandwidth of h D 0:2t 1=6 . In the context of the quantile evaluations, this choice of bandwidth provides reasonable performance for various underlying distributions in our extensive MC study. In addition, the choice of bandwidth satisﬁes the asymptotic conditions on required in Chen & Hall (1993) (see also Yu et al., 2011, for details). 2.2. Derivation of the functional form of prior information via the local maximum likelihood estimation based on statistical tables ® 1˛ ¯ In this section, we use the table qn;m I n 2 N; m 2 M; ˛ 2 A to deﬁne a prior distribution on 0 qn1˛ . Consider the model 0 ;m0 1˛ qn;m D .˛; n; m/ C "1˛ n;m ; "1˛ n;m N.0; 1/; (10) rn X rm r˛ X X iD0 j D0 kD0 RE .˛; n; m/ Š CT ED where ./ is an unknown function corresponding to location and > 0 is an unknown scale parameter. We estimate the function in order to provide the prior location .˛0 ; n0 ; m0 / 0 for qn1˛ , the parameter of interest. In order to estimate .˛0 ; n0 ; m0 / and , we propose 0 ;m0 directly applying the LML methodology (e.g. Fan et al., 1998). The LML methodology is a nonparametric maximum likelihood technique based on a polynomial-type approximation of a parameterized function of interest, where the parameterized function is often unknown. The point of interest in our procedure is the point .˛0 ; n0 ; m0 / for which the corresponding quan® 1˛ ¯ tile is not tabulated in the table of critical values qn;m I n 2 N; m 2 M; ˛ 2 A . In the LML approach, we approximate the parametric function via Taylor expansion as ˇijk .˛ ˛0 /i .n n0 /j .m m0 /k ; ˇijk D ˇikj : (11) CO R The restriction ˇijk D ˇikj corresponds to a wide class of two sample hypothesis tests, including the applications considered in Section S1 of the Supporting information. This condition can be avoided in general evaluations. Equation (11) allows an approximation to the location parameter around the point of interest .˛0 ; n0 ; m0 /. The ultimate parameter of interest is ˇ000 D .˛0 ; n0 ; m0 / in the polynomial approximation, while incorporating all available data points into the parameter estimation. Let 0 be the standard deviation corresponding to (the estimator of) ˇ000 . ˇijk and are estimated through maximizing the log likelihood given by L.“; ˛0 ; n0 ; m0 / D log UN 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 A. Vexler et al. p 1 2 2 2 2 X k.˛ ˛0 ; n n0 ; m m0 / n2N;m2M;˛2A X 0 @q 1˛ n;m n2N;m2M;˛2A r˛ X rn X rm X 12 i j kA ˇijk .˛ ˛0 / .n n0 / .m m0 / i D0 j D0 kD0 k.˛ ˛0 ; n n0 ; m m0 /; (12) where “ indicates the vector composed of ˇijk and k./ is a joint kernel function. We write k˛0 ;n0 ;m0 D k.˛ ˛0 ; n n0 ; m m0 / in order to ease the notational burden of this formulation. The joint kernel function k˛0 ;n0 ;m0 deﬁnes the contribution of tabulated data points to the log-likelihood (12), thus yielding a local kernel weighted log-likelihood function. The kernel function centred around the values of interest .˛0 ; n0 ; m0 / provides a larger weight © 2014 Board of the Foundation of the Scandinavian Journal of Statistics. Computing critical values of exact tests Scand J Statist for the data points closer to the values of interest relative to those points more distant. Without compromising this concept, we deﬁne the joint kernel function as k˛0 ;n0 ;m0 D kh1 .˛ ˛0 /kh2 .n n0 /kh3 .m m0 /; 1 2 PR OO F where khi ./ D k.= hi /= hi is a univariate density function with khi .x/ D khi .x/ for all real x and bandwidth hi ; hi > 0. Typically, the ranges of tabulated n and m are similar so that we let h2 D h3 . The values of r˛ ; rn ; rm and the bandwidth are selected on the basis of the bias and variance estimates of the nonparametric estimator (Fan et al., 1998). As building blocks of the bias and variance estimation, we note that @L.“; ˛; n; m/[email protected]“ is a vector whose elements corresponding to ˇi 0 j 0 k 0 are given as 9 28 r˛ X rn X rm = < X 1˛ i j k 4 q ˇijk .˛ ˛0 / .n n0 / .m m0 / ; : n;m n2N;m2M;˛2A i D0 j D0 kD0 3 X i0 k0 5 j0 k˛0 ;n0 ;m0 ; ED .˛ ˛0 / .n n0 / .m m0 / (13) ı and @2 L.“I ˛; n; m/ @“2 is a matrix whose elements are given as CT @2 L.“; ˛; n; m/ ; @ˇi 0 j 0 k 0 @ˇi j k with diagonal elements corresponding to ˇi 0 j 0 k 0 , 2 0 (14) P 0 .˛ ˛0 /2i .n n0 /2j 0 n2N;m2M;˛2A n2N;m2M;˛2A RE .m m0 /2k k˛0 ;n0 ;m0 and off-diagonal elements corresponding to ˇi 0 j 0 k 0 and ˇi j k , P 0 0 0 2 .˛ ˛0 /i Ci .n n0 /j Cj .m m0 /k Ck k˛0 ;n0 ;m0 . Analogous to ´ CO R the univariate approach of Fan et al. (1998), the bias for the vector of estimators “O can be estimated by @2 L.“; ˛; n; m/ μ1 @“2 ˇ ˇ @L.“; ˛; n; m/ ˇˇ ˇ @“ ˇ (15) O “D“ given values of r˛ , rn and rm . If we set .˛; n; m/ D .˛0 ; n0 ; m0 /, then on the basis of the results in (13) and (14), the bias deﬁned at (15) becomes 0 for each component. Thus the bias is not of concern when we obtain the likelihood function estimates at the value of interest, namely .˛0 ; n0 ; m0 /. This result follows immediately because the Taylor expansion is carried out around the point .˛0 ; n0 ; m0 / so that the ﬁrst term of (11) always corresponds to the true value of ˇ000 regardless of the order of the expansion. Hence there is no bias involved in the estimation of ˇ000 . Analogous to Fan et al. (1998), the variance estimators “O are obtained in the form of UN 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 9 ´ O ˛; n; m/ D var.“; @2 L.“; ˛; n; m/ @“2 μ1 ² @L .“; ˛; n; m/ var @“ ³´ @2 L .“; ˛; n; m/ @“2 μ1 ˇˇ ˇ ˇ ˇ ˇ O “D“ (16) © 2014 Board of the Foundation of the Scandinavian Journal of Statistics. 10 Scand J Statist In (16), because elements of the vector, var ¹@L.“; ˛; n; m/[email protected]“º corresponding to ˇi 0 j 0 k 0 , have the form 8 9 r˛ X rn X rm < = X X 1 2 1˛ i j k k˛0 ;n0 ;m0 var qn;m ˇijk .˛ ˛0 / .n n0 / .m m0 / 2 : ; iD0 j D0 kD0 n2N;m2M;˛2A .˛ ˛0 / 2j 0 .n n0 / 2k 0 .m m0 / ; F 2i 0 PR OO (17) the only term not to be 0 is the element for ˇ000 , when evaluated at .˛; n; m/ D .˛0 ; n0 ; m0 /. Thus, on the basis of (14), (15) and (16), (17) reduces to ˇ ˇ ® 1˛ ¯ˇ P 2 k˛0 ;n0 ;m0 var qn;m ˇ000 ˇˇ n2N;m2M;˛2A ˇ var.ˇO000 ; ˛0 ; n0 ; m0 / D : (18) ˇ !2 ˇ P ˇ ˇ k˛0 ;n0 ;m0 ˇ n2N;m2M;˛2A O “D“ ED 0 Thus, we have a prior for q01˛ D qn1˛ N 0 ; 02 , where 0 D ˇ000 .˛0 ; n0 ; m0 / 0 ;m0 and 2 D var ˇO000 . The values of r˛ , rn and rm are obtained to minimize 2 D var ˇO000 . 0 0 CT We employ the optimal bandwidth hi on the basis of minimum variance (Simonoff, 1998, p. 105). In practice, the values of r˛ , rn and rm also can be assumed to be ﬁxed, say, to be 2 or 3. 2.3. The procedure to calculate critical values of exact tests incorporating Monte Carlo simulations and statistical tables CO R RE In this section, we provide the algorithm for executing the proposed method in practice. The procedure is based on the following steps: (a) Obtain the prior distribution with the parameters 0 ; 02 using the LML method on the basis of tabulated critical values. This prior has the form deﬁned in Section 2.2. (b) Generate a learning sample (e.g. deﬁning t D 200/ of the test statistic values under the corresponding null hypothesis using MC simulations. (c) Using the learning sample, estimate f q01˛ f 0 q01˛ f 00 q01˛ and f .3/ q01˛ to Q present E .t ˇ / of ˇproposition 2 as a function of t. Q /ˇ with a presumed threshold (e.g. D 0:001/ to compute an appropri(d) Compare ˇE .t ˇ ˇ Q 0 /ˇ < . ate value of t0 when ˇE .t (e) Run the MC simulations to obtain t0 values of the exact test statistic. (f) Use (8) or its asymptotic form from proposition 1 to obtain the estimator of the quantile of interest, evaluating the critical values. UN 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 A. Vexler et al. Remark 4. The proposed estimator given at (8) or its asymptotic form from proposition 1 can be used to directly evaluate the critical values given a ﬁxed number of MC simulations (e.g. t0 D 10; 000/. In this case, proposition 2 provides the method from which we can measure the accuracy of the given estimation procedure. Remark 5. Classical hypothesis testing methodology offers to preselect the signiﬁcance level of the test by deﬁning the corresponding critical value of the test statistic. The test statistic is then compared with the critical value with the possible decisions of rejecting the null hypothesis H0 or not rejecting H0 . An alternative approach to hypothesis testing is based on computing the © 2014 Board of the Foundation of the Scandinavian Journal of Statistics. Computing critical values of exact tests Scand J Statist 0 (19) PR OO 1p T0 D @qO M 10 11 1 0 A @1 C A ; C 1p 1p t t 02 Bt2 qO M 02 Bt2 qO M F p-value (e.g. Gibbons & Chakraborti, 2011). Generally speaking, the p-value is a value of a uniformly distributed random variable that can measure the smallest signiﬁcance level at which the null hypothesis would be rejected conditionally on the observed data. The procedure we have developed can be easily modiﬁed for evaluating p-values. For example, using proposition 1, we can deﬁne p as a root of CT ED where T0 represents a value of the test statistic computed conditional on the observed data. 1p That is, qO M can be computed as a function of T0 , and then by virtue of the deﬁnition1 p D P t 1p Xj , we obtain a value of p that gives an approximation to the true t 1 j D1 Kh qO M underlying p-value. Similarly, but slightly more complex, one can use (8) to obtain a pvalue approximation. Several other procedures to examine the p-value, on the basis of the proposed method, are possible. Algorithms employed to develop the software packages of Tanajian et al. (2013) and Shepherd et al. (2013), for example, consist of the following steps: (1): (a), (b) and (c) as mentioned earlier with q01˛ D T0 ; (2) compute the value of p using (19); and Q /j with a presumed threshold . If jE .t Q /j , we approx(3) taking ˛ D p, compare jE .t Q imate the p-value using p. If jE .t/j > , then we generate more test statistic values under the corresponding null hypothesis using MC simulations, thus obtaining t D t C d , where, for example, d D 200, and go to Step (1.c). 3. Concluding remarks CO R RE In this paper, we deﬁned and examined the nonparametric posterior expectation of quantiles within the general setting of hypothesis testing. The goal of the proposed approach was to develop a general framework for evaluating the p-values of a variety of exact tests, incorporating the relevant MC simulations and statistical tables in a Bayesian framework as observed and prior information, respectively. We used the smoothed EL function (Chen & Hall, 1993) to encapsulate information from the MC experiments, whereas tabulated critical values were used to compute prior distributions. The new method improves the schemes used to calculate critical values of exact tests. We showed two asymptotic propositions to conduct exact tests using asymptotic critical points and provided the rule to select the appropriate number of MC simulations needed for a desired degree of accuracy. The asymptotic result we developed is useful in practice for evaluating critical values of interest. The MC simulation studies demonstrated that the proposed method yields very accurate estimators for the critical values of exact tests. It should be noted that the proposed method can also be applied to power calculations (Cohen, 1988). The proposed method is also very ﬂexible in terms of the variety of applications. For example, Tanajian et al. (2013) employed the main idea of this paper to develop STATA statistical software packages for testing symmetry and for K-sample comparisons. In a subsequent paper, we plan to address the use of the bootstrap and asymptotic Wilks-type propositions to achieve accurate estimation of critical values. Further studies are needed to evaluate the approach in other contexts. We hope that this paper will stimulate future theoretical and applied research on this topic. UN 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 11 Appendix: The proofs of propositions 1 and 2 R 1˛ To prove proposition 1, we calculate the integral EL.q/.q/dq in the neighbourhood of qO M 1˛ for large t , where qO M is a maximum of the smoothed EL function, EL.q/. The approach © 2014 Board of the Foundation of the Scandinavian Journal of Statistics. 12 A. Vexler et al. Scand J Statist UN CO R RE CT ED PR OO F for evaluating the integral is based on the following stages. The Taylor theorem can be applied 01 to represent approximately the function L./ at (A.1), which is the ﬁrst derivative of EL.q/ 02 with respect to the Lagrange multiplier . The obtained Taylor approximation ofL./ at (A.5) 03 can be used in the evaluation of on the basis of the observed data points. This form can 04 yield values of the integrals of interest. Thus, we begin with a lemma about the existence of the 05 maximum value of the smoothed EL function in the convex hull of ¹X1 ; : : : ; Xt º. The lemma 06 shows that the behaviour of the EL is similar to that of parametric likelihoods with respect to 07 increasing/decreasing of the likelihoods and their maximum occurrences. 08 09 1˛ Lemma 1. The smooth EL function, EL.q/, deﬁned at (2) monotonically decreases if q > qO M , 10 1˛ 1˛ and monotonically increases if q < qO M , where the maximum of EL.q/ occurs at q D qO M that 11 1˛ P satisﬁes 1 ˛ D t 1 tj D1 Kh qO M Xj . 12 13 Proof. By virtue of (2) and (3), the derivative of the log EL.q/ is 14 15 t Y @ @ 1 16 log EL.q/ D log @q @q t C ¹K .q Xj / .1 ˛/º h 17 j D1 18 @ t ¹Kh .q Xj / .1 ˛/º C kh .q Xj / X @q 19 D t C ¹Kh .q Xj / .1 ˛/º 20 j D1 21 t X kh .q Xj / 22 D ; t C ¹Kh .q Xj / .1 ˛/º 23 j D1 24 this follows from the fact that is a root of 25 t X Kh .q Xj / .1 ˛/ 26 D 0: 27 t C ¹Kh .q Xj / .1 ˛/º j D1 28 Thus, the sign of @ log EL.q/[email protected] depends on the sign of .q/. In order to obtain the sign of 29 .q/, we deﬁne the function 30 31 t X Kh .q Xj / .1 ˛/ 32 L./ : (A.1) t C ¹Kh .q Xj / .1 ˛/º j D1 33 34 1˛ Note that @L./[email protected] < 0, and hence L./ is decreasing with respect to . Because qO M 1˛ Pt 35 1˛ 1 satisﬁes t O M Xj .1 ˛/ D 0, at the point q D qO M is 0, that is, j D1 Kh q 1˛ 36 qO M D 0. In this case, pj , j D 1; 2; : : : ; t , in (8) are just t 1 , and the constraint, 37 Pt j D1 pj ¹Kh .q Xj / .1 ˛/º D 0, is automatically satisﬁed. 38 Consider the two situations: 39 40 1˛ i) If q < qO M . If .q/ D 0, then 41 t t ± 42 1X 1 X° 1˛ L..q/ D 0/ D ¹Kh .q Xj / .1 ˛/º Kh qO M Xj .1 ˛/ D 0; 43 t t j D1 j D1 44 because Kh is a smooth function belonging to .0; 1/. 45 1˛ We are interested in 0 such that L.0 / D 0. So, we have 0 < 0, for q < qO M . 46 The details of this case are depicted in Fig. A.1, taking the function, L./, into account 47 FA.1 which decreases when increases. Therefore, because @ log EL.q/[email protected] > 0, log EL.q/ 48 1˛ monotonically increases for q < qO M . 49 © 2014 Board of the Foundation of the Scandinavian Journal of Statistics. Computing critical values of exact tests PR OO CT ED 1˛ Fig. A.1. The function L./ is plotted against for the case of q < qO M to evaluation of the sign of 0 such that L.0 / D 0. RE 1˛ Fig. A.2. The function L./ is plotted against for the case of q > qO M to evaluation of the sign of 0 such that L.0 / D 0. 1˛ ii) If q > qO M . Likewise, we have t t ± 1 X 1 X° 1˛ ¹Kh .q Xj / .1 ˛/º Kh qO M Xj .1 ˛/ D 0: t t CO R L¹.q/º D j D1 j D1 1˛ We have 0 > 0, for q > qO M , shown in Fig. A.2. L./ decreases as decreases. 1˛ In a similar manner to the case, q < qO M , because @ log EL.q/[email protected] < 0, log EL.q/ 1˛ monotonically decreases for q > qO M . The proof of lemma 1 is complete. Pt To simplify the proof of proposition 1, let us deﬁne kNh .q/ D j D1 kh .q Xj /=t and ® ¯s Pt KN s;h .q/ D j D1 Kh q Xj .1 ˛/ =t for s D 1; 2; 3. UN 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 13 F Scand J Statist Proof of proposition 1. To prove proposition 1, we ﬁrst note that p 1˛ q OM 't = X Z .t/ Z EL.q/.q/dq D Z EL.q/.q/dq C X.1/ 1˛ q OM 't = X Z .t/ 1˛ q OM C't EL.q/.q/dq; p = t © 2014 Board of the Foundation of the Scandinavian Journal of Statistics. t EL.q/.q/dq p X.1/ C p 1˛ q OM C't = t t (A.2) FA.2 14 Scand J Statist given 't D t ˇ , 0 < ˇ < 1=2. By lemma 1, we have p 1˛ q OM 't = .p i Z1 h 1˛ expŒlog EL.q/.q/dq exp log EL qO M 't t .q/dq t X.1/ 1 h .p i 1˛ D exp log EL qO M 't t : (A.3) F Z L./ D KN 1;h .q/ PR OO In order to show that the upper bound at (A.3) vanishes to 0, we consider the function L./ deﬁned in (A.1). This function can be rewritten as t X ¹K.h1 .q Xj // .1 ˛/º2 : 2 t 1 C t ¹K.h1 .q Xj // .1 ˛/º j D1 (A.4) Deﬁne 0 D t =gt with gt D t . Then, deﬁning 2 .0; 1=2/, one can show that p t L.0 / D p t KN 1;h .q/C t t X gt p j D1 tX gt t j D1 ¹K.h1 .q Xj // .1 ˛/º2 1C 1 gt ¹K.h1 .q Xj // .1 ˛/º ED p p p t L.0 / D t KN 1;h .q/ ¹K.h1 .q Xj // .1 ˛/º2 1 1 gt ¹K.h1 .q Xj // .1 ˛/º ! 1 and ! 1; as t ! 1; RE CT p because t KN 1;h .q/ D Op .1/ (Chen & Hall, 1993). Thus, we obtain the order D O.t=gt /, where is the solution to L./ D 0. In this case, because KN 4;h .q/ D Op .1/ by Chen & Hall (1993), the Taylor’s expansion of the function L./ with respect to around 0 provides the approximation ! 2 3 : (A.5) L./ D KN 1;h .q/ KN 2;h .q/ C 2 KN 3;h .q/ C Op t t t3 D CO R Solving (A.5) for gives t KN 1;h .q/ C Op KN 2;h .q/ t gt2 (A.6) ; which gives gt D o.h1 /; that is, if h D o.t 1=6 /, then 2 .0; 1=6/. Because KN 3;h .q/ D Op .1/ by Chen & Hall (1993), the Taylor expansion of the function log EL.q/ when is considered around 0, and (A.6) implies that ! ! 2 3 t¹KN 1;h .q/º2 3 log EL.q/ D KN 1;h .q/C KN 2;h .q/COp D : (A.7) CO p 2t t2 t2 2KN 2;h .q/ UN 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 A. Vexler et al. 1˛ 1˛ By the Taylor expansion of the function (A.7) around qO M at q D qO M 't 1˛ Op .1/ and KN 2;h qO M D Op .1/ (Chen & Hall, 1993), we obtain ! ® 1˛ ıp ¯2 .p 't t t kNh qO M 't3 1˛ 1˛ log EL qO M 't C Op 1=2 t D t 2KN 2;h qO M ! 3 't D C 't2 C Op 1=2 ! 1 as t ! 1; t © 2014 Board of the Foundation of the Scandinavian Journal of Statistics. ıp 1˛ t , kNh qO M D (A.8) for some constant C > 0. Therefore, by virtue of (A.3) and (A.6), we conclude that p 1˛ q OM 't = Z t EL.q/.q/dq D op 1 : t (A.9) X.1/ EL.q/.q/dq D op 1˛ q OM C't p = t 1 : t PR OO X Z .t/ F Similarly, we also obtain (A.10) Equations (A.9) and (A.10) imply that (A.2) can be rewritten as 1˛ q OM C't = X Z .t/ Z EL.q/.q/dq D X.1/ p t EL.q/.q/dq C op 1˛ q OM 't p = t 1 : t (A.11) ED Through the result at (A.7), the right hand side of (A.11) can be written as p t p 1˛ ¯2# qO M ZC't = t " ® N t K1;2 .q/ 1 1 EL.q/.q/dq C op exp D .q/dq C op : N 2;h .q/ t t 2 K p p 1˛ 1˛ q OM 't = t q OM 't = t (A.12) Z CT 1˛ q OM C't = Hence, we approximate (A.12) as the function 1 " ® ¯2 # t KN 1;2 .q/ 1 exp : .q/dq C op t 2KN 2;h .q/ RE Z1 CO R ® ¯ q By the assumption of .q/ D exp .q 0 /2 =202 = 202 and the Taylor expansion of the q 1˛ function KN 1:h .q/= KN 2;h .q/ with respect to q around qO M , we have " .q 0 /2 t¹KN 1;2 .q/º2 exp 202 2KN 2;h .q/ # UN 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 15 Computing critical values of exact tests Scand J Statist 3 ° ±2 1˛ N 2 / .q 1 2 7 6 t kh qO M 0 1˛ 1˛ q qO M D exp 4 5 C op 2 t 20 2KN 2;h qO M 2 2 61 D exp 4 2 ´ 1˛ 1 C 2 tBt2 qO M 0 tBt2 1˛ qO M μ8 < 1˛ qO M 1˛ 1 q tBt2 qO M C 2 : 0 0 C 2 0 ! 92 = ; 3 7 5 C op !1 1 ; t 1˛ ® 1˛ ¯2 . 1˛ where Bt2 qO M D kNh qO M KN 2;h qO M . Thus, the asymptotic estimator of the nonparametric posterior expectation of the quantiles is 1˛ qO NP D 1˛ qO M C 1C 0 1˛ 02 Bt2 .q OM /t 1 1˛ 02 Bt2 .q OM /t C op 1 : t This completes the proof. © 2014 Board of the Foundation of the Scandinavian Journal of Statistics. (A.13) 16 Scand J Statist 1˛ To prove proposition 2, we start with the fact that qO M satisﬁes the relationship 1 ˛ D 1˛ Pt t q O . It follows that the Taylor expansion of the function around q01˛ K X j j D1 h M provides the approximation 2 KN 1;h q01˛ kNh0 q01˛ 1˛ 1 1˛ 1˛ qO M D q0 : (A.14) 1˛ 1˛ qO M q01˛ C op N N t kh q0 2kh q0 Note that we have covariance expression D 1 cov t 1 k k q01˛ X1 h f .q01˛ /.1C2˛/ t Co .1 ˛/ PR OO cov kNh q01˛ ; KN 1;h q01˛ D F 1 ;K 1 t q01˛ X1 h : : (A.15) 1˛ 3 1˛ Proof of proposition 2. Because qO M q0 D oP .t 1 / from ® 1˛ ¯3 2ı D Op h t and (9), we obtain the following equation as KN 1;h q 0 1˛ qO NP qO 1˛ q01˛ D˛ f NP 2 1 1 Q C op ˛ .t/ C op : t t 1˛ qO NP q01˛ q01˛ ED 1F 2 (A.14) and f 0 q01˛ (A.16) CT Equations (A.14) and (A.16) jointly imply that 2 1 1˛ .0 q01˛ / .0 q01˛ / 1˛ f 0 .q 1˛ / qM q01˛ C 2 B 2 qO 1˛ qM q01˛ C 2 B 2 qO 1˛ t . / 0 t M B 0 t . M / t C 1˛ Q .t/ [email protected] / : f .q A 2 1 C 2 B 2 1qO 1˛ t 0 t . M / 2 1 C 2 B 2 1qO 1˛ t 0 t . M / 1˛ Because Bt q0 be rewritten as 0 B Q .t/ [email protected] Df (A.17) ı 1˛ q01˛ ˛.1 a/ C op .1/ and qO M D q01˛ C op .1/, (A.17) can 2 CO R 2 RE 0 1˛ qM q01˛ C 1C ˛.1˛/.0 q01˛ / 02 f 2 .q01˛ / t ˛.1˛/ 02 f 2 .q01˛ / t 1 1˛ 1˛ 2 f 0 q 1˛ 1 C 1˛ qM q0 : Af q 2 C op t ˛.1˛/ 2 1 C 2 f 2 q 1˛ t . 0 / 0 (A.18) Q We take the expectation of the function .t/ in (A.18) using (A.14). Then, we obtain UN 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 A. Vexler et al. 0 1˛ q01˛ C E qM 1 1˛ 1˛ 2 f 0 q 1˛ 1 C 1˛ E qM q0 Co f q A 2 ˛.1˛/ t 1 C 2 f 2 q 1˛ t ˛.1˛/ . 0 / 0 2 1 C 2 f 2 q 1˛ t . 0 / 0 0 1 N 1;h .q 1˛ / K ˛.1˛/f 0 .q01˛ / ˛.1˛/.0 q01˛ / 0 E C 1˛ 1˛ 2 2 1˛ 3 N B kh .q0 2f .q0 0 f .q0 / /t /t C C DB @ A 1 C 2 f˛.1˛/ 2 .q 1˛ / t B Q E .t/ [email protected] ˛.1˛/.0 q01˛ / 02 f 2 .q01˛ / t 0 ˛.1˛/f 0 .q 1˛ / f 2 .q01˛ / t f .q 1˛ / 2 1C ˛.1˛/ 02 f 2 .q01˛ / t 0 2 C o 1 : t (A.19) © 2014 Board of the Foundation of the Scandinavian Journal of Statistics. F2 Computing critical values of exact tests Scand J Statist ı To compute E KN 1;h q01˛ kNh q01˛ , we use the Taylor expansion in two variables of f .x; y/ D y=x given as y Ey Ey 1 .x Ex/ C .y Ey/ x Ex .Ex/2 .Ex/2 3Ey 1 C .x Ex/2 .x Ex/.y Ey/: .Ex/3 .Ex/2 (A.20) F f .x; y/ D PR OO Equations (A.15) and (A.20) imply that ! KN 1;h q01˛ 2 f 0 q 1˛ h2 4 f .3/ q01˛ h4 1˛ E D k 01˛ C k 1˛ 2f q0 24f q0 kNh q0 2 2 0 1˛ 00 1˛ 4 f q01˛ .1 C 2˛/ f q0 f q h 1 1˛ 0 Co : k t t 4f 2 q0 The proof of proposition 2 is complete. ED Acknowledgements CT This research is supported by the NIH grant 1R03DE020851 - 01A1 (the National Institute of Dental and Craniofacial Research). The authors are grateful to the editor and the referee for suggestions that led to a substantial improvement in this paper. References CO R RE Chen, S. X. & Hall, P. (1993). Smoothed empirical likelihood conﬁdence intervals for quantiles. Ann. Stat. 21, 1166–1181. Cohen, J. (1988). 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