The budget balance can be decomposed into a cyclical and a non-cyclical, or structural, component. The decomposition is aimed at separating cyclical influences on the budget balances resulting from the divergence between actual and potential output (the output gap), from those which are non-cyclical. Changes in the latter can be seen as a cause rather than an effect of output fluctuations and may be interpreted as indicative of discretionary policy adjustments. It should be noted, however, that changes in resource revenues -- as a result of oil price changes, for example -- and in interest payments -- as a result of past debt accumulation or changes in interest rates -- are neither cyclical nor purely discretionary. Yet these changes are reflected in the evolution of the structural component of the budget balance.