  We revisit estimation and computation of the Dickey Fuller (DF) and DF-type
tests. Firstly, we show that the usual one step approach, based on the "DF
autoregression", is likely to be subject to misspecification. Secondly, we
clarify a neglected two step approach for estimation of the DF test. (In fact,
we introduce a new two step DF autoregression.) This method is always correctly
specified and efficient under the circumstances. However, it is either
neglected or misused in unit root testing literature. The commonly employed
hybrid of the (correct) two step method is shown to be inefficient, even
asymptotically. Finally, we further improve/robustify the proposed two step
method by employing the missing initial observations. Our finally proposed
method is to be used in unit root testing, since it is a new DF autoregression
that retains the missing observations.
