In microeconomics, the long run is the conceptual time period in which there are no fixed factors of production, so that there are no constraints preventing changing the output level by changing the capital stock or by entering or leaving an industry. The long run contrasts with the short run, in which some factors are variable and others are fixed, constraining entry or exit from an industry. In macroeconomics, the long run is the period when the general price level, contractual wage rates, and expectations adjust fully to the state of the economy, in contrast to the short run when these variables may not fully adjust.
In the long run, firms change production levels in response to (expected) economic profits or losses, and the land, labour, capital goods and entrepreneurship vary to reach the minimum level of long-run average cost. In the simplified case of plant capacity as the only fixed factor, a generic firm can make these changes in the long run:
The long run is associated with the long-run average cost (LRAC) curve in microeconomic models along which a firm would minimize its average cost (cost per unit) for each respective long-run quantity of output. Long-run marginal cost (LRMC) is the added cost of providing an additional unit of service or commodity from changing capacity level to reach the lowest cost associated with that extra output. LRMC equalling price is efficient as to resource allocation in the long run. The concept of long-run cost is also used in determining whether the long-run expected to induce the firm to remain in the industry or shut down production there. In long-run equilibrium of an industry in which perfect competition prevails, the LRMC = LRAC at the minimum LRAC and associated output. The shape of the long-run marginal and average costs curves is determined by returns to scale.
The long run is a planning and implementation stage. Here a firm may decide that it needs to produce on a larger scale by building a new plant or adding a production line. The firm may decide that new technology should be incorporated into its production process. The firm thus considers all its long-run production options and selects the optimal combination of inputs and technology for its long-run purposes. The optimal combination of inputs is the least-cost combination of inputs for desired level of output when all inputs are variable. Once the decisions are made and implemented and production begins, the firm is operating in the short run with fixed and variable inputs.
All production in real time occurs in the short run. In the short run, a profit-maximizing firm will:
The transition from the short run to the long run may be done by considering some short-run equilibrium that is also a long-run equilibrium as to supply and demand, then comparing that state against a new short-run and long-run equilibrium state from a change that disturbs equilibrium, say in the sales-tax rate, tracing out the short-run adjustment first, then the long-run adjustment. Each is an example of comparative statics. Alfred Marshall (1890) pioneered in comparative-static period analysis. He distinguished between the temporary or market period (with output fixed), the short period, and the long period. "Classic" contemporary graphical and formal treatments include those of Jacob Viner (1931),John Hicks (1939), and Paul Samuelson (1947). The law is related to a positive slope of the short-run marginal-cost curve.
The usage of long run and short run in macroeconomics differs somewhat from the above microeconomic usage. John Maynard Keynes in 1936 emphasized fundamental factors of a market economy that might result in prolonged periods away from full-employment. In later macroeconomic usage, the long run is the period in which the price level for the overall economy is completely flexible as to shifts in aggregate demand and aggregate supply. In addition there is full mobility of labor and capital between sectors of the economy and full capital mobility between nations. In the short run none of these conditions need fully hold. The price level is sticky or fixed in response to changes in aggregate demand or supply, capital is not fully mobile between sectors, and capital is not fully mobile across countries due to interest rate differences among countries and fixed exchange rates.
A famous critique of neglecting short-run analysis was by Keynes, who wrote that "In the long run, we are all dead", referring to the long-run proposition of the quantity theory of money, for example, a doubling of the money supply doubling the price level.