The full effect that a change in a causal variable has on the dependent variable. In a regression model, where Y = a + bX, a shift in X has an instantaneous effect (of b) on Y. In dynamic regression, there are lags in either X or Y in the model. A shift in X also has a long-run effect, which may either amplify or damp the short-run effect. When using causal variables in a forecasting model, one is typically concerned with long-run effects. Thus, it is inadvisable to formulate a model on first differences.