Ng and Perron (2001) constructed four unit root test statistics that are calculated using generalized
least squares (GLS) de-trended data for a variable. Compared to the widely-used Dickey-Fuller
(DF) and Phillips-Perron (PP) unit root tests, these have better power and size properties. The first
unit root test statistic developed by Ng and Perron calculates the Elliot, Rothenberg, and Stock
(ERS) point optimal statistic for GLS de-trended data as follows: