Omay, TolgaEmirmahmutoglu, FurkanHussain Shahzad, Syed JawadEconomics2024-07-052024-07-052021100003-68461466-428310.1080/00036846.2020.18149452-s2.0-85093098839https://doi.org/10.1080/00036846.2020.1814945https://hdl.handle.net/20.500.14411/3168Shahzad, Syed Jawad Hussain/0000-0003-3511-6057; Emirmahmutoglu, Furkan/0000-0001-7358-3567We 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.eninfo:eu-repo/semantics/openAccessNonlinear trenddeterministic smooth breakstructural changeestimation methodsArticleQ2537761780WOS:000579891700001