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 sticky-price theory asserts that the output prices of some goods and services adjust slowly to changes in the price level. Suppose firms announce the prices for their products in advance, based on an expected price level of 100 for the coming year. Many of the firms sell their goods through catalogs and face high costs of reprinting if they change prices. The actual price level turns out to be 110. Faced with high menu costs, the firms that rely on catalog sales choose not to adjust their prices. Sales from catalogs will , and firms that rely on catalogs will respond by the quantity of output they supply. If enough firms face high costs of adjusting prices, the unexpected increase in the price level causes the quantity of output supplied to the natural level of output in the short run.