To implement the proposed structural time series (unobserved components) modeling framework, we represent real GDP (Yt) as
Where represents a stochastic trend (unit root) component of real GDP, represents a stochastic (trigonometric) cycle, and , is an innovation. Similarly, we represent the real price of oil as
Where represents the stochastic trend (unit root) component of the real oil price, Prepresents a stochastic (trigonometric) cycle, and , is an innovation. In addition, we specify these models such that the drift elements of both and are themselves random walks, thus allowing for the possibility of time-varying rates of drift (which we interpret as potential GDP growth rates).
Alternative models are constructed by letting represent one or more exogenous variables. 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; may also be a vector consisting of various combinations of the variables listed earlier. The dimension of the vector of coefficients, , is a adjusted appropriately to match that of . Across alternative models, we let be an exogenous (scalar) impulse related to the 1974 oil price shock, that only enters the oil price equation ; represents the coefficient associated with the 1974 oil price shock.