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Article Citation - WoS: 10Citation - Scopus: 10Comparison of Optimization Algorithms for Selecting the Fractional Frequency in Fourier Form Unit Root Tests(Routledge Journals, Taylor & Francis Ltd, 2021) Omay, Tolga; Emirmahmutoglu, Furkan; Hussain Shahzad, Syed JawadWe compare the performance of unit root tests which include flexible Fourier trends in their testing processes. The algorithms considered are those of Broyden, Fletcher, Goldfarb and Shanno (BFGS), Berndt, Hall, Hall and Hausman (BHHH), Simplex, Genetic and grid search (GS). The simulation results indicate that derivative-free methods, such as Genetic and Simplex, have advantages over hill-climbing methods, such as BFGS and BHHH in providing accurate fractional frequencies for fractional frequency flexible Fourier form (FFFFF) unit root test. When the parameters are estimated under the alternative hypothesis of the FFFFF type of unit root test, the grid search and derivative-free methods provide unbiased and efficient estimations. We also provide the asymptotic distribution of the FFFFF unit root test. We extend the FFFFF unit root test to a panel version in order to increase the power of the test. Finally, the empirical analyses of healthcare convergence show that derivative-free methods, hill climbing and extensive grid searches can be used interchangeably. However, for big data and accurate estimation of the frequency parameters, the Simplex methodology using the bootstrap process is preferred.Article Citation - WoS: 35Citation - Scopus: 36Structural Break, Nonlinearity and Asymmetry: a Re-Examination of Ppp Proposition(Routledge Journals, Taylor & Francis Ltd, 2018) Omay, Tolga; Emirmahmutoglu, Furkan; Hasanov, MubarizIn this study, we examine the validity of the PPP proposition for 28 European countries. For this purpose, we propose a new unit root test procedure that allows for both gradual structural breaks and asymmetric nonlinear adjustment towards the equilibrium level. Small-sample properties of the new tests are examined through Monte-Carlo simulations. The simulation results suggest that the new tests have satisfactory size and power properties. We then apply these new tests along with other unit root tests to examine stationarity properties of real exchange rate series of the sample countries. Our tests reject the null of unit root in more cases when compared to alternative tests. Overall, we find that the PPP proposition holds in majority of the European countries examined in this article.Article Citation - WoS: 5Citation - Scopus: 5Nonlinear Error Correction Based Cointegration Test in Panel Data(Elsevier Science Sa, 2017) Omay, Tolga; Emirmahmutoglu, Furkan; Denaux, Zulal S.We propose a nonlinear error correction-based cointegration test in a panel data setting and provide their small sample properties. (C) 2017 Elsevier B.V. All rights reserved.Article Citation - WoS: 3Citation - Scopus: 9Regime Dependent Causality Relationship Between Energy Consumption and Gdp Growth: Evidence From Oecd Countries(Routledge Journals, Taylor & Francis Ltd, 2021) Emirmahmutoglu, Furkan; Denaux, Zulal; Omay, Tolga; Tiwari, Aviral KumarThis study empirically investigates the energy consumption-GDP growth nexus for the period from 1971 to 2016 for 26 OECD countries. The prevailing studies in the literature use limited econometric methodologies, which may wrongly model the underlying relationship and lead to misleading policy conclusions. Our study utilizes the newest econometric methods to reveal the nonlinear relationships in the long-run. Furthermore, to capture the asymmetric behaviour of regime changes, four residual-based nonlinear cointegration tests are implemented. Finally, a two-regime TAR type of panel threshold VECM model (PTVAR) is estimated for testing the presence of nonlinear short- and long-run causality. Our findings indicate a state-dependent causality between energy consumption and GDP growth.
