Bootstrapping unit root tests with covariates

We consider the bootstrap method for the covariates augmented Dickey–Fuller (CADF) unit root test suggested in Hansen (1995 Hansen, B. E. (1995). Rethinking the univariate approach to unit root testing: Using covariates to increase power. \ Econometric Theory 11:1148–1171.[Crossref], [Web of Science ®] , [Google Scholar]) which uses related variables to improve the power of univariate unit root tests. It is shown that there are substantial power gains from including correlated covariates. The limit distribution of the CADF test, however, depends on the nuisance parameter that represents the correlation between the equation error and the covariates. Hence, inference based directly on the CADF test is not possible. To provide a valid inferential basis for the CADF test, we propose to use the parametric bootstrap procedure to obtain critical values, and establish the asymptotic validity of the bootstrap CADF test. Simulations show that the bootstrap CADF test significantly improves the asymptotic and the finite sample size performances of the CADF test, especially when the covariates are highly correlated with the error. Indeed, the bootstrap CADF test offers drastic power gains over the conventional unit root tests. Our testing procedures are applied to the extended Nelson and Plosser data set.     

Nonparametric estimation of regression models with mixed discrete and continuous covariates by the K-nn method

In this article we consider the problem of estimating a nonparametric conditional mean function with mixed discrete and continuous covariates by the nonparametric k-nearest-neighbor (k-nn) method. We derive the asymptotic normality result of the proposed estimator and use Monte Carlo simulations to demonstrate its finite sample performance. We also provide an illustrative empirical example of our method.     

A simulation-based multi-objective optimization framework: A case study on inventory management

We propose a simulation-based solution framework for tackling the multi-objective inventory optimization problem. The goal is to find appropriate settings of reorder point and order quantity to minimize three objective functions simultaneously, which are the expected values of the total inventory cost, the average inventory level, and the frequency of inventory shortage. We develop new algorithms that can exploit statistically valid ranking and selection (R&S) procedures and the desirable mechanics of conventional multi-objective optimization techniques. Two simulation algorithms are proposed to be applied in different scenarios depending on the preference information that is revealed either during or after the actual optimization process. Experimental results are provided to evaluate the efficiency of the developed algorithms and other existing solution frameworks.     

Negative ability bias from conditioning on a confounded mediator: A directed interaction test and case study

Sociologists increasingly recognize the problem of endogenous selection bias in mediation analysis. Building on a sub-model of Elwert and Winship (2014), we propose that a widely-overlooked endogenous selection bias exists in the form of a negative ability bias in sociological models of economic outcomes where job attributes are often included as mediators but confounded by unobserved abilities. We use simulations to show that the bias size increases with collider selectivity under common modeling assumptions, and we leverage this property to develop a directed interaction test to detect this bias. To demonstrate the principles of the test and the practical significance of the bias, we re-examine the paradox of Communist Party Premiums in post-2000 China—the sudden disappearance of income returns to Party members, as found in recent studies. The results are strikingly consistent with the hypothesis that a negative ability bias has recently overwhelmed the persistent Party premiums in China.     

Early social exposure and later affiliation processes within an evolving social network

BackgroundWhile short-term experiments have found that exposure to social contacts affects likeability, this study investigates its long-term effects on friendships in an emerging network.MethodsOne hundred psychology freshmen indicated whether they attended an introductory event before the start of the academic year to assess early exposure and reported on the friendships they formed with their peers at the beginning, middle, and end of their first semester.ResultsRSiena network models revealed that students taking part in the introductory event were more likely to form friendships throughout the first semester, but less likely to keep them.DiscussionWhile early exposure to people in an emerging network might provide a head start for forming friendships for at least a semester, these friendships appear less stable than those of students who did not have that exposure.     

Testing the difference between spectral densities of two independent periodically correlated (cyclostationary) time series models

In this work, the asymptotic distribution for the discrete Fourier transform of periodically correlated (PC) processes is applied to test the equality of two PC time series. Then the performance of the proposed method is investigated through the Monte Carlo simulations.     

Doubly sparse regression incorporating graphical structure among predictors

Recent research has demonstrated that information learned from building a graphical model on the predictor set of a regularized linear regression model can be leveraged to improve prediction of a continuous outcome. In this article, we present a new model that encourages sparsity at both the level of the regression coefficients and the level of individual contributions in a decomposed representation. This model provides parameter estimates with a finite sample error bound and exhibits robustness to errors in the input graph structure. Through a simulation study and the analysis of two real data sets, we demonstrate that our model provides a predictive benefit when compared to previously proposed models. Furthermore, it is a highly flexible model that provides a unified framework for the fitting of many commonly used regularized regression models. The Canadian Journal of Statistics 47: 729–747; 2019 © 2019 Statistical Society of Canada     

Bayesian quantile regression and variable selection for partial linear single-index model: Using free knot spline

Partial linear single-index model (PLSIM) has both the flexibility of nonparametric treatment and interpretability of linear term, yet existing literatures about it mainly focused on mean regression, and quantile regression analysis is scarce. Based on free knot spline approximation, we apply asymmetric Laplace distribution to implement Bayesian quantile regression, and perform variable selection in linear term and index vector via binary indicators. Our approach is exempt from regularity conditions in frequentist method, and could execute variable selection and quantile regression under mutual posterior correction, which is also the first work to implement them jointly for PLSIM in fully Bayesian framework. The numerical simulation manifests the superiority of our approach to previous methods, which embodied in better efficiency of variable selection, index vector estimates and link function approximation with different error distributions. For illustration of its application, we build a power consumption model of A₂/O process in wastewater treatment and emphatically analyze the impact of water quality factors.