Unit roots and purchasing power parity: another kick at the can

Lopez et al. (2005 Lopez, C, Murray, C and Papell, D. 2005. State of the art unit root tests and purchasing power parity. Journal of Money, Credit and Banking, 37: 361–9. Crossref], Web of Science ®] , Google Scholar]) demonstrated that single-equation unit-root tests cannot provide conclusive evidence of whether real exchange rates are stationary because inference depends critically on the lag-lengths used to construct the test statistics, a result reinforced by a recent work by Sweeney (2006 Sweeney, D. 2006. Mean reversion in nominal G-10 exchange rates. Journal of Financial and Quantitative Analysis, 41: 685–708. Crossref], Web of Science ®] , Google Scholar]). The purpose of this article is to revisit the issue, first demonstrating the necessary conditions under which this approach of testing for Purchasing Power Parity (PPP) is appropriate.