random walks, thus allowing for the possibility of time-varying rates of drift (which we
interpret as potential GDP growth rates).6
Alternative models are constructed by letting 5,Y represent one or more
exogenous variables. gY may be a scalar consisting, for example, of the fiscal deficit, the
real exchange rate, real GDP growth in the U.S. and interest rate differentials; 4gY may
also be a vector consisting of various combinations of the variables listed earlier. The
dimension of the vector of coefficients, 8
6 'y, is a adjusted appropriately to match that of
X . Across alternative models, we let U/P be an exogenous (scalar) impulse related to the
1974 oil price shock, that only enters the oil price equation; 3'y represents the
coefficient associated with the 1974 oil price shock.
ECONOMETRIC SPECIFICATION AND RESULTS
Estimation Results
To facilitate the identification of the permanent and cyclical components of GDP
and oil prices, we impose the restriction of common (up to sign and scaling factors) levels
and cycles.7 We have used annual data from 1970 to 2000.8 Several models including
various alternative (exogenous) policy variables have been estimated. The only variable
that has been found to be statistically significant at conventional significance levels in the
presence of real oil prices, is interest rate differentials. Thus we only report two sets of
estimation results for models with and without interest rates differentials. The method of
estimation is maximum likelihood.
The model that excludes interest rate differentials has 12 parameters, with only 2
restrictions.9 Convergence is achieved after 55 iterations.t 0 Summary statistics are
presented in the Table "Venezuela: Trend-Cycle Model of GDP and Oil Prices. Interest
Rate Differentials Excluded". The estimated trend-cycle models have good statistical
properties, with residuals being approximately normal." The estimated model residuals
display little heteroskedasticity and autocorrelation (including at higher orders). It is