In the short run, the discrepancy between actual and expected price level causes changes in output and employment. But in the long run, if all other things remain constant, the higher price level will come to be accurately expected by firms, narrowing down the difference between expected and actual price levels. This is important because in the long run, the costs incurred by business firms rise as economic agents react to higher prices. Wage earners, for example, now pay more for the same basket of goods and services they used to purchase earlier. A hike in wage rates will be bargained for and the same will be reflected in higher prices by the suppliers of goods and services.

As we have seen, the higher level of output in the short run was possible only because the unanticipated rise in the price level led to higher profits to business firms. As soon as the costs increase in line with final prices, the incentive to produce higher levels of output disappears and the production reverts to its original level. In this situation, the level of output will be at its natural rate and deviations from this state are possible only when actual price level differs from the expected price level in the short run. Thus, in the long run, the natural rate of output is the equilibrium rate of output for the economy. As shown in figure 6.5 the short run aggregate supply curve is given by AS_S and AS_L is the long run aggregate supply curve. The level of output is given by Q which is the natural rate of output.