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Discussion Paper

An ADF Coefficient Test for A Unit Root in ARMA Models of Unknown Order with Empirical Applications to the U.S. Economy

This paper proposes an ADF coefficient test for detecting the presence of a unit root in ARMA models of unknown order. Our approach is fully parametric. When the time series has an unknown deterministic trend, we propose a modified version of the ADF coefficient test based on quasi-differencing in the construction of the detrending regression as in Elliot, Rothenberg and Stock (1996). The limit distributions of these test statistics are derived.