Robust Inference for Near-Unit Root Processes with Time-Varying Error Variances |
| |
Authors: | Matei Demetrescu Christoph Hanck |
| |
Institution: | 1. Institute for Statistics and Econometrics, Christian-Albrechts-University of Kiel, Kiel, Germany;2. Faculty of Economics and Business Administration, University of Duisburg-Essen, Essen, Germany |
| |
Abstract: | The autoregressive Cauchy estimator uses the sign of the first lag as instrumental variable (IV); under independent and identically distributed (i.i.d.) errors, the resulting IV t-type statistic is known to have a standard normal limiting distribution in the unit root case. With unconditional heteroskedasticity, the ordinary least squares (OLS) t statistic is affected in the unit root case; but the paper shows that, by using some nonlinear transformation behaving asymptotically like the sign as instrument, limiting normality of the IV t-type statistic is maintained when the series to be tested has no deterministic trends. Neither estimation of the so-called variance profile nor bootstrap procedures are required to this end. The Cauchy unit root test has power in the same 1/T neighborhoods as the usual unit root tests, also for a wide range of magnitudes for the initial value. It is furthermore shown to be competitive with other, bootstrap-based, robust tests. When the series exhibit a linear trend, however, the null distribution of the Cauchy test for a unit root becomes nonstandard, reminiscent of the Dickey-Fuller distribution. In this case, inference robust to nonstationary volatility is obtained via the wild bootstrap. |
| |
Keywords: | (Near-)Integrated process Nonlinear instrument Normality Time-varying variance |
|
|