Is Germany's GDP Trend-Stationary? A Measurement-With Theory Approach

The time series properties of German GDP have been re-examined in recent research. Extending the sample to include GDP data from 1950 onwards, some researchers argued in favor of a trend-stationary rather than difference stationary representation of real log GDP. I show that this conclusion is based on an atheoretic trend model underlying the unit root tests. A simple linear trend model fails to take the post World-War II catch-up process properly into account. I use the Solow growth model to discriminate between transitional catch-up dynamics and long-run equilibrium growth. With the proper transformation of GDP data, I am able to use standard unit root tests and find that both ADF and KPSS tests suggest a difference stationary model. This evidence is supported by non-standard unit root tests which allow for polynomial trend representations.