Why the aggregate supply curve slopes upward in the short run In the short run, the quantity of output that firms supply can deviate from the natural level of output if the actual price level in the economy deviates from the expected price level. Several theories explain how this might happen. For example, the misperceptions theory asserts that changes in the price level can temporarily mislead firms about what is happening to their output prices. Consider a soybean farmer who expects a price level of 100 in the coming year. If the actual price level turns out to be 90, soybean prices will , and if the farmer mistakenly assumes that the price of soybeans declined relative to other prices of goods and services, she will respond by the quantity of soybeans supplied. If other producers in this economy mistake changes in the price level for changes in their relative prices, the unexpected decrease in the price level causes the quantity of output supplied to the natural level of output in the short run. Suppose the economy's short-run aggregate supply (AS) curve is given by the following equation: Quantity of Output Supplied Quantity of Output Supplied = = Natural Level of Output+αÃ( Price Level Actual â Price Level Expected ) Natural Level of Output+αÃPrice LevelActualâPrice LevelExpected The Greek letter α α represents a number that determines how much output responds to unexpected changes in the price level. In this case, assume that α=$4 billion α=$4 billion . That is, when the actual price level exceeds the expected price level by 1, the quantity of output supplied will exceed the natural level of output by $4 billion. Suppose the natural level of output is $40 billion of real GDP and that people expect a price level of 100.