This paper proposes new unit root tests for panels where the errors may be not only serial and/or cross- orrelated, but also unconditionally heteroskedastic. Despite their generality, the test statistics are shown to be very simple to implement, requiring only minimal corrections and still the...

This paper considers the issue of whether shocks to ten commodity prices (gold, silver, platinum, copper, aluminum, iron ore, lead, nickel, tin, and zinc) are persistent or transitory. We use two recently developed unit root tests, namely the Narayan and Popp (NP, 2010) test and the Liu and...

In this paper, we test the efficient market hypothesis for 100 US firms listed on the New York Stock Exchange. To test the unit root null hypothesis, we develop a generalized autoregressive heteroskedasticity (GARCH) model that not only caters for the GARCH errors but also allows for two...