In economic moels, the long-run time frame assumes no fixed factors of production. Firms can enter or leave the marketplace, and the cost (and availability) of land, labor, raw materials, and capital goods can be assumed to vary. In contrast, in the short-run time frame, certain factors are assumed to be fixed, because there is not sufficient time for them to change. This is related to the long run average cost (LRAC) curve, an important factor in microeconomic models.